The quest can be found and started on the Sanctuary III in Claptraps room.
After picking it up wait a second until the small box on the table opens up as it will gift you 500 Eridium for free. Next head to Darkthirst Dominion and simply follow the quest marker.@evomat91 told us in the comments that
"If anyone is having trouble with this go into the invincible game with level 11 mayhem start the fight then turn the mayhem of. It says will reload achievement then unlocks".-> As several people have confirmed it looks like it works on the second attempt. Please look in the comment section below to find out if the trick still works and share your experience. I will try to update the guide frequently.
-> I still recommend doing this the normal way with just killing him.NOTE:
To fight against this Boss you will have to pay 500 Eridium - that means if you die you have to pay the 500E again in order for another try. You will face a huge door which only opens once you payed the price.NOTE2:
@SideKick Dan pointed out that after respawing the door was still open. He only had to repay after he left the level to grabed some different weapons and came back.
I suggest to disable any Mayhem/Chaos mode and play on normal difficulty.
After entering the arena you will have to kill a few mobs and Hemovorous the Invincible
will mutate a few times before you will face his final form and the real fight begins.
The Boss himself got two health bars:
The first one is yellow (Corrosive will melt through Armor).
The second one is red (Incendiary damage tears through Flesh).
Regarding his attacks I suggest to not stand still and always keep moving while shooting.
You can dodge some of the attacks with moving/jumping over it.
Down below you can find a helpful video and I suggest you to watch it before fighting him by yourself.
I killed him solo in my first try and the achievement popped straight after he died but only two seconds later I died too and wasn't able to loot him
.Video Credits: Griffins Gaming Guides
Good luck fellow Vault Hunters!