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 Baddass, then Super, then God Raging Goliath and get larger each time. He'll need to kill 10 enemies (your enemies, his friends) to get to max level, he'll always be called a God at that point.

Note they're *much* tougher after they've leveled up, but they drop accordingly better loot, particularly at God level.