I did the achievement on co-op by having another account go out and get all the kills. My primary account did not die, and no achievement popped up. What went wrong?
I did not get the achievement when I didn’t die and my co-op partner died once, (right next to the index!).
Therefore, I believe neither player in a co-op session is allowed to die, otherwise no one gets the achievement.
EDIT: Someone below said their co-op partner died whilst they didn’t, and they did get the achievement, so maybe it’s a bugged.
I have it now though, after a run in co-op where neither of us died.
Guardax were you playing splitscreen by any chance?
i was playing co-op with a friend and he died, i didn’t, i still got the achievement…