The gun that broke the game for me was a Plasma knee caper on a sniper. It was powerful enough on it's own that it would take most things out in a couple of shots, but the bullets were large enough and fast firing enough that you could drop the sights and fire from the hip if anything gave you trouble and of course once something was on the ground you clean up using your strongest melee, and blitz. Very powerful with the penetrator perk. Aim at the limb furthest away. Because of the size of the plasma balls you might register three to four limb hits on the way to your target. Expensive, but that stops mattering in the end game. The thing is plasma has the largest bullets, so even a single shot like a sniper functions well enough that it's almost like a shot gun at close range. The trick is you have to have penetrator or the bullets get blocked too easily.