My friend and I have killed Donkey Mong three times now, but have yet to get the achievement for it on either of our accounts. Does anyone know if we need to be playing single-player for this achievement now? I did look through the archived posts and saw that someone else had had the same issue. Everyone responded stating that he was probably killing King Mong and not Donkey Mong, but I can assure you that we've killed Donkey Mong thrice (not only was he throwing barrels at us, but his name also distinctly said "Donkey Mong").
The first time we killed him, we were in vehicles. Thinking this may have been the reason we didn't get it, we hopped out of our cars the second time and killed him with our regular tactics, which included elemental weapons. Thinking that maybe it was elemental damage that killed him (instead of a direct shot/hit from us), we decided to use non-elemental weapons the third time...and still we didn't get it.
Now, we're not farming him or anything, so I can't say we've tried everything. I was just wondering if anyone knows if this is glitched in split-screen, because we're thinking of trying it out solo next. (We've been waiting to hit level 61 before finally playing these characters solo so that we can be the same level at all times.)
Cutting his tie is not part of the achievement, but the BA Challenge. I also know that when coop killing only the person who lands the final blow gets credit for the kill (everyone still gets XP of course). At least, that's how it worked with the 'Goliath, Meet David' Achievement. We had 4 people in the group and actually did that one but only one of us got the achievement.
Those factors aside, it was probably just a glitch. Either way, Donkey Mong is easy to spawn solo by entering from the Hero's Pass area vice the normal Fast Travel location. He usually spawns 30% of the time when entering from Hero's Pass. If it doesn't work, just save/quit and try again.