this is one way to get rare guns.

first you find a goliath then you shoot the head off the goliath and it should turn into a raging goliath. then run towards an enemy (best enemys are nomad or pyro nomad)the raging goliath should attack the enemy/s. once its killed one it should level up keep repeating this until the goliath turns into a god-liath then kill it and a rare item or gun should drop.