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