According to the in-game achievement list the is one more hidden achievement in Doomsday Coop. The hint is: Level Clonetron in coop. Second last entry. Can't remember to have seen it mentioned here. Any hints on this one?
The Doomsday achievement is probably bugged. It's not hard to get. *Hint hint* It's just "No one dies".
Not so sure about that, as already posted. Not hard to get? Umpf. Tried for an hour yesterday, to no avail. At least I got the 15 clones.