If you're crafty with a sniper rifle, this achievement is for you. In order to get 'Bullseye', you have to take out all of the bad guys in the level "Drop 'em Like Liquid" without alerting anyone to your presence. Good luck with that, by the way. If you manage to pull this off, you'll be rewarded with 20G on Xbox 360, or a bronze trophy on PlayStation 3.
Here's a great tutorial video made by YouTuber XBLStallion83, which you can find below. Now I really want to put more time into the single player campaign.