Like most bosses, use your surroundings to your advantage. With General Knoxx-trap, it's best to use a Revolver or SMG (depending on skill level) and continually circle around him while shooting at his head. It's pointless to hide behind anything since he's so slow. Additionally, claptraps will spawn and attack as well. Thankfully, the map is open wide so you can move around freely. There are 2 areas you fight Knoxx. The trophy will unlock the second time you fight him (on the way to fighting the final boss going through the wayward pass).
Happy Hunting :)