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