Here is my Guide for this Achievement.
Donkey Mong is a extremly rar Monster in SW of Eridium Blight.
The "Fastest" Way to let him spawn is a Quit - Reload Session. Travel with fast Travel to Eridium Blight, get a Car and search the Spawnpoint
Is Donkey Mong not here, quit the Game and Continue, get the Car and search the Spawnpoint and again and again until he spawns.
I know its a sick methode but i think its the fast!
Question? ask me :)
PS: sry 4 english, german ;)