Borderlands

Achievement Name: Knoxx-Trap
During mission "Operation Trap Claptrap Trap, Phase Four: Reboot", you will enter into 3 separate arena battles. Defeating the first boss will unlock this achievement.

If you are having trouble with Knoxx (I did after I finished the second playthrough of the story and his level boosted)

This method works very well with a soldier, using the crate when you enter the arena as cover, draw knoxx to the right hand side, he cannot get through the gap, allowing you to critical hit over and over with your best weapon, every so often, Kamikazee clap-traps spawn, because knoxx is blocking the right side, the clap-traps will all go left, so just throw down your scorpio turret to take them out, and continue firing on knoxx. As long as you keep an eye on when the clap-traps spawn, you will be fine.

if you want to do this achievement, a really easy way is to get a second controller and play split screen with your character and a completely new one, and make sure the new character is the host. So player 1 will be level 1 and you will be player 2 with whatever high level you are. Then complete all the missions. This achievement is the last mission. But because player one is the host all of the enemies in the game will be a really low level, so you should be able to shoot them one shot. Youll finish the DLC is no time! and because your such a low level, the knoxx-trap will also be a low level so he will be really easy to kill.