You don't have to do this in the same game, but its probably easier. Just keep researching experimental Heavy weapon and experimental powered weapon in the Proving Grounds until you have a Blaster Launcher, Flamethrower, Hellfire Projector, Plasma Blaster, Shredder Gun, Shredstorm Cannon, and Rocket Launcher (unlocked by default on your heavy armor platforms.) Then just land a killing blow on an enemy with each of these weapons.