Originally Posted by Son of Kyuss
Both players, if you're playing co-op, can't die at all or neither of you will get the achievement.
Now the reason why 343i made it like this was probably because they wanted it to be an actual challenge rather than have people piggyback in co-op to unlock it.
Yeah I figured as much and I saw it right after posting this too. Might need to update my "never again" chievo