Location; Eridium Blight
Donkey Mong;
Have to Kill for achievement only!King Mong; Don't have to kill for achievement but he is an easter egg and will drop loot!
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I CAN confirm this it unlocks for all 4 players! Even 2! and Single Player!*
1. From the fast travel in Eridium Blight grab a catch a ride car and head south.
2. Head through the gates where the turrets are.
3. A big monkey might spawn in front of you on the right THIS IS NOT DONKEY MONG this is King Mong and he doesn't give you the achievement but can give you loot if you wish to kill him. If you are under level this will be a tough battle.
4. Keep driving until you reached the spot on the map above or video below. It should be on the right.
5. He should spawn out of one of the holes on the wall.
6. If he doesn't spawn the first time save and quit and go back to Eridum Blight.
7. Rinse and Repeat until he spawns may be your first or second try or more but he will show up keep trying.
8. For fun you can cut his tie by meleeing it for a challenge.
Enjoy this entertaining video the buddies at RoosterTeeth made all credit goes to them for finding (Cursing warning);