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