After you break out of your cell and steal the weapons, you'll have to clear two corridors of Russian soldiers. After this, a helicopter will fly over and you'll reach a courtyard. The sniper will have a laser sight which makes it easy to know where hes targeting. There's not much cover on the left so stick right-mid while clearing out enemies. It is very difficult to rush the warehouse, so I used an RPG I found near the top of the stairs leaning against a table on the right side. Shoot it at the sniper, kill him, and the achievement should pop after a few seconds. You can get it by killing him when reaching the warehouse, but I found the RPG easier. The normal soldiers can hit you, it's only the sniper you have to avoid.