Army of Two (EU)

Simply kill 5 enemies while in Co-op snipe.

Co-op snipe is a scripted event, so you can only enter it at certain parts. When instructed, or when you otherwise see a co-op snipe marked target on your GPS, activate co-op snipe through the co-op partner menu. For this to work you must have a sniper rifle, as opposed to a rocket launcher or stinger missle launcher.

In this mode, the computer NPC will shoot when you shoot, allowing for simultaneous shots.

The easiest way to get this achievment is in the first level, when you are being instructed by Clyde. When he tells you to go into co-op snipe and take out the gas cylinder, instead take out one of the guards nearby, and proceed to kill the rest of the enemies in the area. If needed, rinse and repeat.

Note that if you exit co-op snipe, you can not re-enter.

By the time you are at the Miami mission you should have bought a decent sniper rifle that enables you to successfully get headshots 90% of the time. Once you get off the monorail in this level you're saved at a checkpoint. After the store you walk to the double doors at the end of the platform and are told to enter Co-op Snipe to take out two guards at the tower entrance. Do as it says and kill them...

Then simply restart from checkpoint, skip past the weapons store and repeat until you have the achievement. This method only takes 30 seconds for each of the kills you still need to get up to your required total of 5.