The battle begins with the three units following behind the player in tandem. The player must climb up the Union before it quickly starts to speed up. Falling down to the ground at this point is instant death.
The Unions' tanks first come combined, then separates to attack, hence the name. It can rejoin back to its unified form crushing anyone in the process. To destroy an individual unit, the player must first shoot its core, namely a large red button. Each unit has its own ways of attacking:
- The Head Tank (number 01) has four main attacks. The core located behind it fires energy projectiles. The head will open up to fire a guided missile, but this missile can be shot down or dodged. The launcher on its sides launches ballistic missiles that will fall to the ground and explode. There is also an energy mortar on the back platform. A way to destroy this unit is to go to the core and destroy it with Machine Guns, or use a Shotgun or a Powered-Up Shotgun from the front, the latter being riskier but faster.
- The V.T.O.L. Gunship (number 02) above both units has four main attacks. The core is located on the belly of the gunship. It also fires energy projectiles, so take caution when destroying the core. The door on the front will either launch Pods or fire a minigun closely similar to the Dragon Nosuke, just more rapidly and moves up and down quickly. When all segments join together, the laser gun on the tail of the gunship will fire. Use a long-ranged weapon for this fight, or go under it and shoot it. It is more accurate but riskier as the VTOL may crush the player.
- The SLAVE Tank (with a number 03) has only two main attacks. The core located in front also fires energy projectiles and multi-cannons will open up on its sides firing bombs in the air similar to the bombs the Hi-Do uses. It is easy to destroy this unit as firing on it will just require the player to stand on the Head Tank's platform. Just be careful when the units join up.
When a unit is destroyed, it will explode and become invulnerable (its color drastically changes). The "destroyed" unit can still attack and unite with active units. When all units are "destroyed", the Union accelerates forward and explodes, with the player(s) jumping off before it does so.
In Other Games
The Union makes its playable appearance in the "Agent Sally" event. It spawns in the battle arena with all of its units joined together making it invincible. Once close to an enemy unit, it loses its invulnerability as the units separate and start to attack.
|Union 03 Ver. PM: Code name "Deneb", the SLAVE Tank. Like the original SLAVE Tank, "Deneb" shoots parachute bombs from a side launcher, though it's core has been modified to shoot a powerful laser beam at its opponents. It is manned by Dragunov.|
|Union 01 Ver. PM: Code name "Vega", the Head Tank. It fires both ballistic and guided missiles. It is piloted by Sisilia.|
|Union 02 Ver. PM: Code name "Altair", the V.T.O.L Gunship. It can strafe enemies with its Vulcan cannon and also shoots energy projectiles. It is piloted by Veronica.|
|Golden Union 03 Ver. PM: Code name "Arided", it is a golden version of "Deneb" whose bombs generate supplies and the core has been modified to shoot out an extremely strong and piercing laser. It is manned by a Special Forces soldier of the Ptolemaic Army.|
|Space Union 01 Ver. PM: Code name "Alnilam", a special unit of "Vega" that fires ballistic missiles and can defend itself with a cosmic energy barrier. It is manned by a Special Forces soldier of the Ptolemaic Army.|
|Union 02 Ver. Rebel: Code name "Naglfar", it is a customized variant of the V.T.O.L Gunship built by the Rebel Army. Like the original V.T.O.L. Gunship, "Naglfar" fires bullets from a vulcan cannon. It can also shoot bombs and has a turret that shoots out a powerful laser beam at its opponents. It is piloted by Chloe.|
|Union 03 Ver. Rebel: Code name "Naglfar", it is a customized variant of the SLAVE tank, though it still retains its signature laser beam. Chloe still pilots the Naglfar, though she sports a different uniform.|
|Frozen Naglfar: A frozen variant of the Union 02 Ver. Rebel Naglfar, operated by a Rebel soldier. The laser cannon has been replaced with an ice beam to freeze enemies in their tracks.|
- This boss resembles the Compiler, the fifth boss from the game R-Type. Possibly an influence as certain members from Metal Slug once worked for Irem.
- The Head Tank in particular seems to resemble the first boss from Gunforce II.
- This is the only boss in the series to be able to split into different parts during a fight.
- The original Ptolemaic Union units are named after the stars that make up the Summer Triangle asterism. Golden Union "Arided" is an older alternative name for "Deneb". Space Union "Alnilam" is instead based on a star from the Orion constillation.