Goliath, Meet David.
Allowed a Goliath to level up four times before killing him.

Goliath's are big hulking monstrosities that can be found throughout Pandora.

The Goliath starts out with a mask/helmet that helps control his rage. First you'll need to shoot this off and at the point of doing so, the Goliath will go into a rage mode. In this, he'll attack anything and everything in sight.

Each kill the Goliath makes while raging will gain him experience points and then levels, becoming even more dangerous. This means you'll need to leave him to rage until he's gone up 4 levels to a total level of 5. You'll know when he's levelled up as he'll do a small animation, gain some health back and his name will change.

Level 0: Goliath
Level 1: Raging Goliath
Level 2: Badass Goliath
Level 3: Super Badass Goliath
Level 4: Ultimate Badass Goliath
Level 5: Fatal GOD-liath
Once he reaches Fatal GOD-liath, you're free to kill him for the achievement.