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