Borderlands: Game of the Year Edition
80 Achievements
1,750
60-80h
United We Stand
Defeated the Rakk Hive, the Vault Boss, Sledge, Krom, or Flynt in a co-op game Details for this achievement will be revealed once unlocked
35
22.46%
How to unlock the United We Stand achievement in Borderlands: Game of the Year Edition - Definitive Guide
Krom is an easy one, as he isn't moving and shoots from his turret. To kill him, simply kill all enemies that you encounter on your way and clear the whole area, so that only he is left.
After that you may be lucky to stay next to a metal wall and have a small visual field through the peephole where you can use your sniper or pistol (or any weapon) with visor.
Be careful, Krom might shoot his turret to you and you get massive damage nevertheless.
When Krom dies you should get your achievement right away. You do NOT need to complete the quest, just kill the enemy.
If you're playing with a friend(s), this trophy will unlock naturally for you. If you are playing solo, an easy workaround is to log in with a second account/controller in split-screen and kill one of the required bosses.
Thanks to Kuolemanlaakso for confirming the method below doesn't work with this version of the game.
Edit: And again, didn't work. I think you need to change your wording, because i'm fairly sure that both accounts need to be there for the killing process. I have done what you suggest for Sledge and Krom, and no trophy.
This can be unlocked with a second account and controller. A silver account will suffice.
The first chance to get this will be against Sledge, however his high powered shotgun, the small area you're in with him and your relative low level at the time of fighting him coupled with the increase in boss health and damage by having a second player present can make this somewhat challenging, especially to those new in the game.
You should then aim to unlock this fighting against Krom since he is in an open area that provides lots of cover and you can take your time making your way towards him, picking off his minions as you go. Krom too is somewhat weak and can even be killed from a distance with a sniper rifle by destroying the turret he is on.
Whoever you defeat in co-op is irrelevant however, as long as you beat one of the bosses named.
The description for this trophy has the bosses in the wrong order of appearance, the correct order is as follows: Sledge, Krom, Flynt, Rakk Hive, Vault Boss. You only have to kill one of these bosses to get the trophy, not all of them and it doesn't matter if you do it online or off. See the individual "Wanted" and "Destroyed" trophies for strategies on how to defeat each boss. It may also be possible to get this trophy by revisiting one of the above bosses you've already killed as most will respawn after saving, exiting and reloading your save (the sole exception is the Vault Boss, because he stays dead once killed). Whichever boss you kill for this trophy, it will pop the moment the boss dies.
It is possible to get this trophy without actually fighting one of the bosses. You can just join an online game where someone has already killed one of them and has the mission ready to Turn In and the trophy should pop (credit to BigDfan228 for this info). Do note though that this trophy may not pop if you kill the Destroyer whilst in Splitscreen and that Sledge can be buggy and might not always pop the trophy.