Borderlands (JP)

You can play this game in either split screen co-op or xbox live co-op so if you don't know anyone else with the game but have a second controller you're in luck.

Now the first boss that you'll encounter that is tied to this achievement will be Sledge, if you're using the splitscreen method you do not need to move the character up to the fight they can stay at the location entry point (which is advised as they'll be far too underleveled)

Now simply kill Sledge, the Rakk Hive, the Vault Boss, Krom, or Flynt whilst in co-op and this achievement will pop.

If you've passed any of these bosses you can return to the area where they were the first time and they'll have respawn for you to kill again, which should work since you do not need to turn the missions in for the achievement

If you have a partner this will come through natural progression in the main storyline. If you are billy no mates like me hook up a second pad, select splitscreen, sign in guest or second profile and enter game.

Leave you 2nd pad idle while you kill one of the bosses. When you finish just turn the mission and the achievement will unlock.