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 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.
Kill 2 enemy with one grenade.
Complete Act 3.
Kill a total of 30 enemies with Browning gun.
Complete Act 2.
Kill 10 birds.
Kill 10 enemies with throwing knives.
Shoot 5 cable lines using sniper rifles.
Kill all enemies attacking your pontoon from the riverside on 'Weaken The Regime' level.
Deliver 100 headshots.
Kill 2 enemy with one shot.
Shoot an enemy through a wall.
Kill 15 enemy using concentration mode.
Complete Act 1.
Deliver 50 headshots.