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