"Kill Greedtooth" is a mission objective in the quest Dwarven Allies in Tiny Tina's Assault on Dragon Keep DLC
Head to the South, out of Ingot Processing then break off to the East when you can. There is a pair of Rock Golems blocking your way forward. Fight on through them and get through the door. Head on forward and drop down to the level below. You will find Greedtooth on the far side of a river of molten lava. He will taunt the Vault Hunter then jump into the lava. He will emerge as a massive Gold Golem that you now need to fight.
This massive Gold Golem fights fairly similar to the Rock Golems. He will hurl stones at you, use a Smash attack which hits everyone nearby him, and throw his massive fists at you. If you are hit by the stones they will daze the Vault Hunter, slowing you down. He will get support in the form of Dwarves and Golems. There are some Aerial Golems that will appear and blast you from above so you need to watch for them as well.
After that cross over the bridge that rises out of the lava and go to the left of the throne to grab the final Dwarven letter.
Enter the arena:
Tina: "You enter the lair of Greedtooth the Dwarf. The Sorcerer brainwashed him into running this place. You also notice a weird-ass magic globe. Might be why Greedtooth's so mean."
Approach Greedtooth: Greedtooth: "Welcome to my lair. Now FRIGGIN' DIE!"
Brick: "A boss fight? Let's do this!"
Tina: "Great job! You have alll the letters. Now just get back to Claptrap."
Lilith: "We gotta walk all the way back? Aw, man."
Tina: "Alright girl, alright. Here -- I made some glowy portals for your laz-a-buttz."
Lilith: "Did you just say 'laserbutts'?"
Brick: "Whatever that is, I want it."
Lilith: "R-A-T-F? We have to unscramble that?"
Brick: "Is it 'raft'? I bet it's 'raft'."