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 pistol. (Multiplayer)
Win 50 rounds playing online. (Multiplayer)
Kill 250 people using sniper rifle. (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.
Shoot 5 cable lines using sniper rifles.
Kill 2 enemy with one shot.
Kill a total of 30 enemies with Browning gun.
Complete Act 2.
Kill all enemies attacking your pontoon from the riverside on 'Weaken The Regime' level.
Complete Act 1.
Deliver 100 headshots.
Kill 10 enemies with throwing knives.
Deliver 50 headshots.
Shoot an enemy through a wall.
Kill 2 enemy with one grenade.
Kill 15 enemy using concentration mode.
Kill 10 birds.