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 sniper rifle. (Multiplayer)
Kill 250 people using pistol. (Multiplayer)
Collect all secrets.
Finish the game on Hard difficulty.
Finish the game on Normal difficulty.
Complete Act 4.
Finish the game on Easy difficulty.
Complete Act 3.
Shoot 5 cable lines using sniper rifles.
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.
Kill 2 enemy with one shot.
Deliver 100 headshots.
Deliver 50 headshots.
Kill 2 enemy with one grenade.
Kill 10 birds.
Shoot an enemy through a wall.
Kill 15 enemy using concentration mode.
Win 50 rounds playing online. (Multiplayer)