With at least 1 other player, kill any of the aforementioned bosses. Specific strategies for each boss can be found on their pages.
- It is also possible to 'glitch' this achievement by killing a boss and then, without turning the mission in, saving and quitting then opening a new game (split-screen or online) where the character that just defeated the boss is the host.
- This achievement is hidden in the Steam version of the game, and won't appear in a Personal Stats page until unlocked.
- From the phrase "United we stand, divided we fall."
- The achievement picture of a handshake is a common way of representing unity or friendship.