Army of Two (EU)

To unlock this achievement you simply need to get 100 kills with sniper rifles. This is a cumulative achievement, so doesn't need to be done all in one go.

If however you want to get this done as quickly as possible, then get to the final area of the first mission (Somalia). Here your partner will be downed and you are supposed to drag them round the corner and revive them. Drag your partner round the corner and instead of healing them, just keep killing the enemies that infinitely respawn. You have unlimited ammo and can just continuously rack up kills.

The snipers in this game are:
-M14
-MSG-90
-DSR-1
-SVD
-M107 Light 50

(If you are going to vote negatively then please post a comment explaining why so I know how to improve the solution)

Quick video guide for the start of Mission 5:




Equip a sniper:
MSG-90
M14
DSR-1
M107
SVD

Fire on the enemies.

Load checkpoint.