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.