Originally Posted by RBmaster9345
Apparently if you're going for the no-death library chievo with co-op if either dies then no unlock. Played through just now and my buddy died right before the end and I didnt get it. Anybody knows whats up here?
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.