The battle starts as the three units follow you from behind and immediately separate to attack you. They first come combined, then separates to attack, hence the name. To destroy it, the player must first destroy its cores, namely a large, red 'button'. Each unit has its own ways of attacking.
The Head Tank (with the number 01 on the front) 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 a 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 Element 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). Keep your guard up however, as while it's "destroyed", it 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.
|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 only fires its guided and ballistic missiles. It is piloted by Sisilia.|
|Union 02 Ver. PM: Code name "Altair", the V.T.O.L Gunship. It only fires its Gatling gun (knockback with Skill 3) and energy projectiles (stun with Skill 4). It is piloted by Veronica.|
|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 manned by Chloe.|
|Union 03 Ver. Rebel: Code name "Naglfar", it is the same as the Deneb of thr Ptolemaic Army, except it still uses its disintegration laser beam as its special attack. Pilot is still Chloe, except she sports a dark gray uniform.|
In Other Games
The Union appears as a Day Specific Mission boss in Metal Slug Attack appearing on Tuesdays-Wednesdays. This unit became playable in the "Agent Sally" Extra Ops. Each section of the Union has its own base health, similar to Crablops.
The Union is invincible until it is separated by letting a unit be close to it. Its movement speed is drastically increased when split. Each of the Units has its own special attack and skills. Unit #01 can knockback enemies, Unit #02 has a chance of getting a critical attack, and Unit #03 can stun enemies. All of them can also perform a suicide attack without any skills required.
The Union's sections were eventually made into separate units, each one with a visible cockpit and pilot.
- This boss resembles the fifth boss from the game R-Type. Possibly an influence as certain members from Metal Slug once worked for Irem.
- This is the only boss in the series to be able to split into different parts during a fight.
- It seems to resemble the chopper boss from Gunforce 2, albeit, being land based and it being a tank than a chopper.