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