The list is pretty stale and boring. But if there isn't one that makes me rage. It's the godamned Overshadowing achievement.
You have to Be RAAM
At least 1 player on the other team has to have the DLC
You have to hope to god that the game doesn't glitch and say you joined in progress and you spawn as your character
You have to hope to be on locust
you have to hope for one other player to be on CoG with the DLC
You have to hope that other people don't deny you and kill you before you get your execution.
Absolute Horseshit. And it's supposed to be done legit. It's like expecting players to execute 500 nemesis