Here is the full list of all 24 Sniper: Ghost Warrior achievements.
Kill 2000 enemies. (Multiplayer)
Collect all secrets.
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)
Finish the game on Hard difficulty.
Complete Act 4.
Finish the game on Easy difficulty.
Finish the game on Normal difficulty.
Kill 10 birds.
Complete Act 3.
Shoot 5 cable lines using sniper rifles.
Kill 2 enemy with one shot.
Complete Act 2.
Deliver 100 headshots.
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.
Deliver 50 headshots.
Kill 2 enemy with one grenade.
Shoot an enemy through a wall.
Kill 15 enemy using concentration mode.