When you shoot off their helmets they start to Rage. You have a few seconds to kill them (don't, if you want the achievement) and then they turn into Raging Goliaths (their health refills too). Raging Goliaths will kill anything near them, **including allies**. Stop shooting him once he starts raging and he should ignore you and go for whoever's closest.

Piss off a Goliath who's with a bunch of buddies from a long range; he'll start slaughtering them. You can wound his buddies to help him, but don't kill them. He'll kill normal Bandits with ease, but you might want to soften up Badasses for him. As he kills them he'll power up and turn into a Super Raging Goliath and apparently even higher levels. Not sure how many enemies you need to get this achievement though.

Note they're *much* tougher after they've leveled up, but they should drop accordingly better loot.