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