For every weapon you find during the game, there is an achievement awarded when you kill thirty enemies with that weapon. However, some weapons are weaker than others (for example, the flamethrower) and are much harder to complete. The easiest way to get these achievements is to find a Guardian (fat blob on the wall that shoots out pods) and kill its pods. Each pod counts as a kill, and it will keep re-spawning pods as long as you don't kill it. You can get all the thirty kill trophies from one Guardian as long as you have enough ammo.