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