So, after 20 runs or so I finally managed to run into Donkey Mong for that achievement "Definitely not an Italian Plumber" or whatever, So I killed him and got no achievement? This really annoyed me since It's taken me ages to find him. It was also on split-screen so I'm wondering if that had anything to do with it?
You know Donkey is the smaller guy that throws barrels, right? NOT the giant one between the first gate and hero's pass, he is moreso on the way to ore chasm. I never looked for him until the other day I was purposely seeking him out and ran into him, achieve popped up fine
"My game is like the Pythagorean Theorem; there is no answer." -Shaq
Follow the strat in the 2nd paragraph on this page for Donkey Mong (except you don't actually have to go back in to Ore Chasm each time, just save, quit, reload, and check again): http://borderlands.wikia.com/wiki/Donkey_Mong
If he's not there by the time you'd jump down the cliff, save and quit. You'll have him spawned within 10 min, tops.