RAGE

The easiest map for this achievement is actually Unwanted Guests. (FYI there is no map called The Dam). Hang back and use the machine gun from range. After clearing an area, look around for all the hidden caches of grenades and wingsticks. Throw EMP grenades into every lasered off, weird floor section before going in. Thrown EMP grenades whenever you see the blue shield enemies (to disable shield) or aim for the hole in their shield. Be sure to use bandages or just pull back and wait to auto heal. This is one of the few maps with NO time limit, and NO swarm of baddies in a confined space.

I also suggest either a local co-op split screen partner for constant communication or proper communication via headset. If you do not communicate one of you WILL go down, even on Normal!

I guess it depends on your skill level but I played on the first prison level on normal, split screen, with a dead controller (I kept getting kamikaze partners who kept running in 'n' dying :( ) (+ this had worked for me previously on 'Splinter Cell' Co-op on realistic so I thought I'd give it a go) this way you have ALL the weapon pickups (make sure you pick up the pop rockets in the store room inside an electricity box where you first encounter the enemies for the x3 big guys with the flame thrower things! (found saving the VX2 bullets handy for these also), door grinder is in a box at the bottom of the stairs outside where the grindable door is) try and save as many of your grenades for the final room if you can and spam the room to kill or get them out of hiding before sniping.
When the mele weapon wielding guys run towards you I found quickly getting the shotgun and crouching to be advantageous in case they ducked just before reaching you and they bore the full brunt of the shot, failing that keep the Wingsticks handy and smash one into his face if you get hit.
Once you've switched the drill off run back up to the office so you can limit the number of direction you can be attacked from.
Be cautious and search everywhere for pickups and go back to the last ammo dump if necessary as it's an unlimited supply, there's no time limit so take your time. I did this on my second try (first time I'd backed into a wall instead of a doorway by accident and got caught as I was retreating to regain health)
Special Note: when you're required to move both people up to open the door move the OTHER player up first and put him behind the bunk bed in a crouching position to stay out of trouble Only then should you move your primary player into the room to open the door.