Metal Slug Infinity (also stylized as Metal Slug ∞) is a Tower Defense, Idle RPG developed and published by ekkorr and licensed by SNK for Android and iOS. The game was first released in the Philippines, New Zealand, and Singapore in March 14, 2019 before being released worldwide at April 16, 2019.
Metal Slug Infinity is a 2D tower-defense game similar to Metal Slug Defense and Metal Slug Attack. A team of six units comes from a base, runs from left-to-right eliminating enemy units, and ultimately destroys their enemy base from the right. Should enemies bypass the player's frontlines (or vice versa), then the player's units turn around to attack the intruders. It being an Idle RPG, the player has little-to-no control over the units' actions aside from rescuing prisoners and activating frontal attacks. Instead, the player manages the game's various currencies to level up units and Slugs on the fly.
As the player advances through stages, it will become apparent that they're overwhelmed, cannot level up quickly, and slowly stops progressing. Metal Slug Infinity has the Return function, where the player uses the Time Slug to backtrack a number of stages. All units have their level reset, all Gold Mines are reclosed, and the player's gold amount disappears. On the upside, returning boosts the units' stats, increases the gold regeneration rate, and rewards Medals, Gems, and Piece Boxes. The commander's level is increased by the maximum stage reached divided by 100 and rounded down (e.g. stage 2780 is level 27).
The game continues on generating gold and units fighting as the player access menus, goes into battles, and closes the app. The units will march offline until they hit the maximum stage the player reached. The player can hit the lock icon to active Screen Saver mode. This lets the units fight in the background at usual speeds. The player exits out by tapping on the unit five times.
Numeration for gold, power, and level amounts are represented in alphabetical order. For example, 1,000 is written as 1a rather than 1k. The next letter occurs every multiple of a thousand (e.g. 1 million = 1b, 1 billion = 1c, 1 trillion = 1d, etc.). After z is aa, ab, ac and so on.
Upon reaching stage 10,000, aside from enemy Regular Army units now appearing, the player enters what is called a Magnetic Storm. The storm lowers battle speed, teleport chance, and stimpack (speed increase) chance by a percentage per stage. For example, for the first Magnetic Storm, stage 12,000 is 20% while stage 18,000 is 80%.
The game uses a multitude of currencies:
- Gold's main purpose is to level up Mines and Units in the main battle arena. Gold slowly increases as time advances, but unlike all other currencies, is reset to zero when Returning. Players can increase the rate of income by opening and leveling up Gold Mines with gold. Certain Gold Mine milestones have a substantial rate increase. The player can call the Genie of Lamp to provide the combined amount of all currently leveled Gold Mines.
- Medals are used to level up the Physical, Flame, and Lightning Type Mastery of all units specific to that type. They can also be used to purchase unit parts in the various shops.
- Gems are premium currency: some amounts are rewarded through daily missions, achievements, stage milestones, and events, and of course they can be directly purchased using real money. Gems are used to enhance units, promote War Books and Artifacts, merge 5+ star Slugs, and level up commander boosts.
- Unit Pieces are parts of a unit needed to unlock that specific Unit or be promoted to the next grade (star) after collecting the required amount, highlighted in blue. Left over Unit Pieces are also currencies for enhancing units and War Books. Piece Boxes provide unit pieces of a certain unit at various amounts up to 1,000 pieces.
- Souls are used to buy Artifact Fragments, Unit Pieces, and EX Slug Boxes at the Unit Soul store. These are obtained by trading excess Unit Pieces of a 10-star unit.
- Honor Coins are rewarded through the main Player vs. Player battles or by daily missions. These are used to buy Rift Detectors at the PVP store, Artifact Fragment at the Artifact store, and enhancing War Books.
- Data Chips are gathered by defeating stages in the Slug Camp. They're used to level up Slugs. A dismantled Slug returns Chips that've been spent on it.
- Quark is used to enhance Slugs. Some amount is awarded whenever S / SR / SSR Slugs are dismantled.
- Infinity Quark allows direct purchases of SSR Slugs through the Infinity shop. It can only be obtained by dismantling SR or SSR Slugs.
- Rift Stones are used to purchase or promote Artifacts. They can be obtained by defeating Artifact stages, or through purchase via Gems.
- And more...
Commander InfoBy clicking on the icon at the upper-left, the player can use their medals to level up the elemental type mastery of all Units of that type. Mastery can be leveled up per one, ten, and a hundred levels at once. If the player changes strategies or now prefers specific types, they can reset the level back to one by a fee of 100 Gems.
Gems are used to level up the player's base. Each of the four facilities have members that boost the percentage per grade by 10% (e.g. 4 Stars is 40%). Each member is progressively unlocked from Lv. 6 to Lv. 10.
- Upgrade Signal Corps: Medals earned are boosted. Its members are Fat Tarma, Dog Mummy, Native, Mummy Fio, and Fat Nadia.
- Upgrade Transportation Corps: March Speed when offline is boosted. Its members are Mummy, Guerrilla, Gatling Soldier, Chowmein-Conga, and Rebel Rifleman.
- Forward Base: The starting stage when Returning is increased. It cannot exceed 60% of the previous Return stage. Its members are Zombie Marco, Tarma-C, Gatling Soldier, Fio-2H, and Mars People.
- Data Chip Research Institute: Data Chip rewards from the Slug Camp are boosted. Its members are Amadeus Soldier, Eri, Donald Morden, Ohumein-Conga (Red), and Special Force.
Other Buffs gives out stats on the Return, Medals earned, Gold Mine boosts, and Offline March Speed and max level. All Buffs is every stat increase through the currently deployed Unit and Slug's abilities, and by the leveled War Books and Artifacts.
Grades and Promotion
The player deploys Regular Army units as well as various friendly enemy Units due to the main cast accidentally transporting themselves to a parallel universe after destroying the Time Slug.
Units have various abilities that are unlocked by leveling up or promoting to raise its grade (number of stars). The color of stars represent the number of overall stars. After five yellow stars, level 6 is one purple star. Unit level is increased by collecting a certain number of Unit Pieces or using Soldier Promotion Tickets.
|8||2100 Pieces or 2 Tickets|
|9||3200 Pieces or 3 Tickets|
|10||4700 Pieces or 4 Tickets|
Unit Pieces are earned by clearing Dungeons, rewards from events and missions, bought through the various Shops, and by purchasing Soldier Boxes at the store. Soldier Promotion Tickets are earned through events or bought at the World Boss store for 3,000 Cores each.
After a unit reaches 10-stars, any excess Unit Pieces can be traded for Souls at a rate of 5:1. Souls can be used at the Soldier Soul store for purchasing Unit Pieces, Artifact Fragments, and EX Slug Boxes.
Abilities and Enhancement
Units have three types of abilities: Level Buff, Possession Bonus, and Skills.
Level Buffs are unit's individual abilities and team boosts that are activated upon reaching level milestones and the unit is part of the active deployment team. The number of skills stops after Lv. 800, and the player can increase the number of ability level milestones by enhancing Units. To enhance a Unit, the player needs Unit Pieces of currently required Units and Gems. An enhanced Unit has a +[number] on the upper right of their icon per enhancement level.
|Lv.||No. of Units||Unit Pieces Each||Gems|
Possession Bonus skills activate regardless if they're deployed or not. Even when deployed, some skills only activate when the unit is present on the battlefield. Possession Bonus abilities are usually related outside of the battle such as skipping stages, raising medal rewards, speeding up battle movement, and increasing Type Mastery Enhancement level. Specific units also boost Gold Mine rewards, increase Warp Energy, increase limit sizes, etc.
Skills are various abilities that activated in battle. These are split into three sub-groups:
- Team Buffs are abilities and boosts that are given to all ally units or those part of a faction. This also includes two battle advancement abilities: Stimpack (increases battle speed) and skipping stages. Each unit has three team buffs that are individually unlocked by reaching a certain grade.
- Battle Skills are individual skills aside from stat boosts. This grants secondary effects to attacks against enemies such as freezing, poison, stun, slow movement, etc. Some units can also jump over obstacles, charge into the action quickly, or teleport at few spaces from the player's base.
- Soldier Buffs are individual timed buffs activated when deployed, after certain time passes, or when the unit is under specific HP. Each unit also has at least one complete immunity to an attack effect.
A deployed Unit and its Slug contribute to the player's Limit Stage. The Limit Stage is the highest stage the player can theoretically reach without hassle. It is counted by the combined total of the Unit's Type Mastery level, its level, its enhanced level, its Slug level, and its Slug enhance level with variance of the War Book's grade. For example: a level 500 +5 Unit deployed with a level 250 +3 Slug while their Type Mastery is at level 1,000 has a Limit Stage of 1758 (500 + 3 + 250 + 3 + 1000).
Units are categorized into three forces:
- Regular Army: Immune to knockback.
- Rebel Army: Includes those from the Ptolemaic Army and Amadeus Syndicate. Immune to shock.
- Another Force: Consists of Martians, Invaders, Mutants, and independents. Immune to stun.
Faction efficiency is as follows: Regular > Rebel > Another > Regular.
Each unit has their own elemental type: Physical, Flame, and Lightning. The color shield a unit deploys with receives immunity for said type color.
Immune to knockback. These units gain an advantage against the Rebel Army, but are weak against Another Force units.
Marco - H
Tarma - C
Fio - 2H
Eri (On the Shore)
XA-01 Juggernaut Marco
Immune to shock. These units gain an advantage against Another Force units, but are weak against the Regular Army.
Immune to stun. These units gain an advantage against Regular Army units, but are weak against the Rebel Army.
Alongside Regular Army vehicles, every unit is able to ride Rebel, Ptolemaic, and Alien machinery regardless on the size of the vehicle or pilot. Units can also ride large mutants such as the Giant Caterpillar. Slugs can be any of the three elemental types. The Unit and Slug's paired Battle Point increases when both are from the same force (e.g. Marco and SV-001; Mortar and Di-Cokka) and same elemental type.
Slugs are leveled up using Data Chips which are earned in the Slug Camp. In the Slug Camp, ten Slugs fight against various units and vehicles as they attempt to destroy the main target. Slugs with a visible pilot are driven by the google-wearing Rebels who fly the R-Shobu.
Unlike the Units, the Slugs do not reset their level upon Returning. Any used Data Chips are returned back to the player for merged and dismantled Slugs. Data Chip rewards are increased by leveling up the Data Chip Research Institute, increasing an Artifact's grade, or by leveling up specific War Books.
Note that assigning a Slug to a Soldier increases that Soldier's stats even when the Slug is not being actively ridden. The Slug's stats act as a multiplier, so it's most effective to pair vehicles with units that have similar strengths. For example, matching a Slug with a high health stat (eg SV-001 Type-R) with a Soldier with a high health stat (eg Fat Tarma) will result in a very tough unit.
Slugs slowly deplete fuel (represented by a green bar) while on the battlefield. Once it loses its health or runs out of fuel, a "TOUCH" icon appears on top. Tapping it allows the unit to send its Slug onto enemy units. Failing to do so simply has the unit safely escape. Frontal attack power is the Unit's attack stat multiplied with the Slug's attack stat.
Slugs also have the ability to Slug Warp; it's accessed by tapping the icon next to the Gold Mine button to swap between Units and Slugs. Slug Warping skips multiple stages at once shown by the arrow symbol below the respective Slug's icon. The battery symbol represents the amount of energy needed to Slug Warp. The player can only Slug Warp at any stages prior to the max stage traveled and cannot Warp at boss stages.
Rarities and Promotion
Slugs come in three rarities: N, R, SR, and SSR. Normal Slugs are weaker or battered variants of higher rarity Slugs. Slugs can be merged (called "promoting") to increase their grade (stars); merging a 5-star N Slug turns it into a 6-star vehicle that is either an R (most likely), SR (unlikely), or SSR (very unlikely). Slugs can be promoted as long as all three are the same Slug and grade. EX Slugs (either bought, randomly excavated, or randomly promoted from a 4-Star Slug) can be used when merging 5-star Slugs of any type. Combining multiple EX Slugs increases the probability of getting an SR or SSR Slug.
Merging N Slugs gives out a random Slug and elemental type. R, SR, and SSR Slugs do not change into other Slugs, and the type a merged Slug comes out is based on the first Slug selected for promotion. N Slugs can be merged for free; the other Rarities require Gems to merge.
R, SR, and SSR Slugs can be dismantled to receive Quark, which is used to enhance Slugs. SRs and SSRs additionally provide Infinity Quark, which allows direct purchases of SSR Slugs in the Infinity section of the Slug store.
The maximum level of a Slug is determined by its grade, rarity, and enhancement level. Furthermore, grade and rarity determines the maximum enhancement level as well. SSR Slugs are hence *significantly* more valuable than R Slugs, as not only are their base stats higher, their level can be increased much further.
The chances of obtaining R / SR / SSR Slugs from a 5-star N merge varies depending on the number of EX Slugs used:
Always merging 3 EX Slugs at a time averages the most SSRs per EX Slug used, even when factoring in SRs dismantled for Infinity Quark. Merging pairs of 2 EX Slugs is not quite as efficient, however, it allows for more Infinity shop purchases - and since such purchases aren't randomised, they can allow for much faster short-term player advancements.
Players interested in long-term advancements should never dismantle SSRs
Aside from Slug Boxes, N Slugs are earned by the Excavation Site. Excavating a site uses every ticket available as long it doesn't exceed the player's Slug limit. Certain units can increase the max ticket account by one once they reach a certain grade.
|Excavation Site||Slug Grade||Soldier||Grade per Ticket|
|Tiny UFO Type-A|
|Zombie (Fattish Man)|
These Slugs can be leveled and enhanced beyond any others. Think very carefully before dismantling them - they're seldom awarded, and all types have their strengths.
Stronger vehicles, although ultimately they can be dismantled for Infinity Quark once enough SSRs have been collected.
Low-value vehicles that can be redeemed for some amount of Quark.
MH-6 s (PM)
Although these are damaged vehicles with very poor battle abilities, they have the potential to evolve to R / SR / SSR forms after repeated merges.
Battered MH-6 s
Battered Mini U.F.O
Battered Hover Unit
Battered T-2B Melty Honey
Battered Bradley Ver.P.M
War Books are individual enhancements for entire factions and types of Slugs. Each book is leveled up with a specific set of colored books (red, yellow, green, and blue) and/or stones. Players earn more books and stones by fighting in the material's respective Boot Camp.
The Boot Camp has a team of six Units and their Slugs destroying a specific Boss while enemy Units protect it. Stages are unlocked by reaching a specific commander level. Later stages put restrictions such as giving the Boss an elemental type immunity and/or disallow an entire faction from participating.
War Books are purchased through the store. Unlike other stores, all items are eventually sold out.
Below are the requirements to promote a current grade War Book.
There are 65 books in total, divided over 13 sets. Equipping each book requires 5 points, and the maximum point limit is increased by using Gems and by promoting certain Soldiers. For 4-book and 6-book sets, every two books equipped awards an additional group bonus. For a 3-book set, all of the books must be equipped for the bonus.
Once all 65 books are promoted to 5 stars, the Enhance menu is unlocked. Here, the player uses Enhancement Stones (earned by daily missions, bought at the World Boss shop for 600 Cores, and one is offered for sale each day for 700 Honor Coins), Honor Coins, and required unit pieces to multiply a War Book's bonuses. A fully enhanced set also multiplies all group bonuses. The game enhances a War Book at random, and there's a 60% chance that two War Books will be enhanced by one stone. The game always picks the first book of the lowest grade and the second one at any grade, and it's not possible to enhance a given book twice until all other books have been enhanced at least once.
- 1st Enhancement: 500 Honor Coins and two Units with 50 Unit Pieces each.
- 2nd Enhancement: 700 Honor Coins and three Units with 50 Unit Pieces each.
- 3rd Enhancement: 1000 Honor Coins and four Units with 50 Unit Pieces each.
Enhancement multipliers are as follow:
|Gold Mine Gold Reward||x200||x50,000||x20,000,000|
|Gold Mine Unlock Decrease||x500||x250,000||x150,000,000|
|Gold Mine Level Up Cost Decrease||x500||x250,000||x150,000,000|
|Gold Mine Time Decrease||x5||x25||x125|
|Soldier Level Up Cost Decrease||x500||x250,000||?|
|Medal Acquisition upon Return||x1.3||x1.6||?|
|Boot Camp Material Reward||x2||x5||?|
|Slug Camp Data Chip Reward||x2||x3.75||?|
Artifacts are icons representing various units. These icons, when promoted, boosts the stats of the related unit (called Pairsoldier [sic]) by ten levels. Aside from boosting the individual unit's stats on the battlefield or at other modes, leveling up its grade also increases Metal counts, Gold Mine rates, and unit affiliated faction stats.
Artifact fragments are bought by using Honor Coins or Rift Stones. Rift Stones are typically earned through the Battle of Rift, by events, or purchased at the different stores.
Artifact fragments are also earned through the Battle of Rift. Unlike other battles, in these missions, a team of 10 Units are positioned at the center while hordes of enemy units gang up on both sides. These enemies become bigger in size as the goal limit is closer to being met. Battle of Rifts are accessed by using Rift Detectors. Stages are unlocked by reaching its specific Artifact Point which are increased by the combined grade of activated Artifacts.
Artifacts are promoted by collecting a number of Artifact fragments and using Gems and/or Rift Stones.
The player fights against an opponent's battle deck. The unit's level is based on the commander level. The battle ends when a player's base is destroyed or all units in a deck are defeated. Gems and Honor Coins are rewarded daily and weekly through rankings. Each battle costs a PVP ticket.
The player attempts to reach a high score by defeating enemies. Unlike other modes, the singular unit is controlled by pressing buttons to attack and dodge, and moving the virtual joystick to move and aim. Unit and Slug Pieces are given per number of attempts. Each attempt is one credit, and more are earned by collecting Arcade Credits.
There are two arcade cabinets:
- Destroy the Subway: Marco or Eri attempt to destroy as many Subways as they can under a time limit while various Rebels fight from both sides.
- Air Strike: The Slug Flyer or Slug Copter fight multiple enemies in the sky.
The player base collaborate together to defeat a specific unit with an abundant amount of health. Each time the boss is defeated, everyone is given two rewards. The player can earn more prizes by giving damage to the boss. A fight against the boss ends when all units are defeated or time ends. The player can access three ally skills and one passive skills to deal more damage.
Ally Skills: Accessed once before regenerating again.
- Defensive Posture: Allies are invincible and have their attacks boosted for a short while.
- Giant Cogwheel: An enormous cogwheel rolls down the arena and damages the World Boss. Allies have their attacks boosted for a while.
- Emergency Slug Call: Everyone enters their Slugs. Ally critical chance is boosted for a while.
- Slug Full Attack: Everyone's Slugs are tossed to the World Boss in which its defenses are reduced for a while.
Passive Skills: Occurs at all times in battle.
- Heal Over Time: A percentage of health is restored every second for all allies.
- Quick Reload: Boosts ally's attack speed by a large percentage.
- Armor Destruction: Decreases the World's Boss' defenses by a large percentage.
- Metal Slug Infinity's promotional material originally had the working title Metal Slug Frontier, a reference to ekkorr's previous idle RPG game, Endless Frontier.