Bunker's tied with Old Slappy for easiest boss in the entire game, man. Just go down to the second level, near the vending machines. The bots rarely come down there, Bunker's cannons don't fire down there, and the mines it launches rarely do, either. I just stand down there and empty clip after clip into it until it dies, easy-peasy. Of course, it'd be a pain getting TO the bunker every time I wanted to farm it, but the bunker fight itself couldn't be much easier. Since I mentioned him, with Slappy, just climb back out of the water and hide behind the big pipe running parallel to the ground. His projectiles can't get past the pipe, but you can shoot at him from the space between the pipe and the ground.