14 Oct 2010
This achievement will pop once you complete the 6 original co-op campaigns (with all objectives, no respawn, and at default+).
Hence, you get this achievement once you get the last achievement of this list :
Station (Co-op 1) Perfect
The Cut (Co-op 2) Perfect
Locks (Co-op 3) Perfect
Caldera (Co-op 4) Perfect
Outpost (Co-op 5) Perfect
Hideout (Co-op 6) Perfect
You DO NOT need to do the extra co-op missions that come with new map packs.
You DO NOT need to do them in one sitting.
Look at each achievement description to know the best strategy for each map. Note that I have done them all with a friend, so they can be completed with only 2 players (even split-screen). In fact, you could try them alone, but some missions require to do things simultaneously...