Union Attacks are a great way to quickly clear groups of enemies on Adventures (so long as the attack is strong enough to K.O. all enemies in one hit), especially if you're using an old monster whose Anima Points will automatically charge the attack, or on high-level Adventures where you gain 600+ Anima on each monster (so you can easily recharge).
Monster Rancher 4Edit
The type of attack is decided by whichever monster is in the front of the formation when you trigger it. If the lead monster is composed, the other monsters will volunteer more readily for the team-up. Likewise, if the monster in the lead is stressed, there is a smaller chance that the other monsters will volunteer for the team up.
Monster Rancher EVOEdit
In this game, you have to "charge" the Union Attack before using it. To charge it, hit the square button while in town or on an Adventure, and choose the Anima Menu. Once there, use the D-pad to select a monster and hit R1 to use that monster's Anima Points to charge the Attack Bar. It costs 100 AP to charge one attack, and each successive charge (up to 9 max) costs double the previous charge.
When you have to have at least 50 Guts and at least one Union Attack charged, you can use a Union Attack (choose the red attack available in the Attack Move Menu. This spends all of your available Guts BUT hits all enemies on the screen. Once you've started your attack, a red/blue bar will appear. You need to hit the X button repeatedly to strengthen your attack. Many Level Bosses also use Union Attacks, which will activate the red/blue bar. You hit the X button in this instance to reduce the damage done by their attack.
Monsters who have aged beyond the point where they gain Anima Points automatically charge the Union Attack with the Anima they would have gained.
Unlike Monster Rancher 4, Union Attacks aren't very useful against bosses because they do fairly small amounts of damage and consume all of your Guts.