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)
Collect all secrets.
Finish the game on Hard difficulty.
Kill 10 birds.
Complete Act 4.
Finish the game on Easy difficulty.
Finish the game on Normal difficulty.
Kill 2 enemy with one grenade.
Complete Act 3.
Kill a total of 30 enemies with Browning gun.
Shoot 5 cable lines using sniper rifles.
Deliver 100 headshots.
Complete Act 2.
Kill 10 enemies with throwing knives.
Kill 15 enemy using concentration mode.
Kill all enemies attacking your pontoon from the riverside on 'Weaken The Regime' level.
Deliver 50 headshots.
Complete Act 1.
Kill 2 enemy with one shot.
Shoot an enemy through a wall.