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There's not much I can offer you in terms of "what you have to do to get this" because, it is clearly stated in the achievement description, however I can offer my advice and I'll link in a video guide too so that you can get a general feel for the level layout and such.

Firstly, this achievement isn't difficulty related, so if you want to nail it quickly go ahead and play on Casual, once you're in the game try and stick in cover and don't go all Rambo on them, even on Casual you have to remember this is a Third Person Shooter and if you keep running straight in you're gonna wind up getting yourself killed, if you're playing co-op teamwork trumps all, if there's a bigger enemy concentrate your fire, he'll drop in half the time.

Now for the videos, it starts off with a small intro interview with Cliff Bleszinski, if you want to skip that part the actual video guide starts at 1:00 on the first video!

*^{Credit to xDYLANxxBAMMx for the videos}