Here is the full list of all 24 Sniper: Ghost Warrior achievements.
Kill 2000 enemies. (Multiplayer)
Kill 500 people using sniper rifle. (Multiplayer)
Kill 250 people using sniper rifle. (Multiplayer)
Kill 250 people using pistol. (Multiplayer)
Win 50 rounds playing online. (Multiplayer)
Complete Act 4.
Finish the game on Easy difficulty.
Finish the game on Normal difficulty.
Finish the game on Hard difficulty.
Collect all secrets.
Complete Act 3.
Complete Act 2.
Kill a total of 30 enemies with Browning gun.
Shoot 5 cable lines using sniper rifles.
Kill 2 enemy with one grenade.
Kill all enemies attacking your pontoon from the riverside on 'Weaken The Regime' level.
Complete Act 1.
Deliver 100 headshots.
Kill 2 enemy with one shot.
Shoot an enemy through a wall.
Deliver 50 headshots.
Kill 10 enemies with throwing knives.
Kill 15 enemy using concentration mode.
Kill 10 birds.