Here is the full list of all 24 Sniper: Ghost Warrior achievements.
Win 50 rounds playing online. (Multiplayer)
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.
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.
Kill 10 enemies with throwing knives.
Deliver 100 headshots.
Kill all enemies attacking your pontoon from the riverside on 'Weaken The Regime' level.
Complete Act 1.
Kill 2 enemy with one shot.
Kill 2 enemy with one grenade.
Deliver 50 headshots.
Shoot an enemy through a wall.
Kill 15 enemy using concentration mode.
Kill 10 birds.