I've done the first 6 levels on legendary co-op. You really don't have to be any good to beat it on legendary. If all else fails, tell one person to stay in cover away from the fight and go in guns blazing until you die. Just do that over and over taking turns until you get it. It might be cheap but if you're desperate you're desperate.
I was playing "The Ark" with a friend that only has the "Needler" achievement and nothing else and we just ended up doing that since it was hard to get him to do any strategic fighting.
[Edited by SuperSkyline89, 1/19/2008 7:13:25 PM]
i've heard of that strategy b4.