Here is the full list of all 50 Spec Ops: The Line achievements. It takes around 15-20 hours to unlock all of the achievements on Windows.
Live and let live.
Complete game on "Walk on the Beach" difficulty.
Complete game on "Combat Op" difficulty.
Carry on, soldier.
The end of the line.
Choose restraint.
Kill 250 enemies with headshots. (campaign only)
Get out of here!
Kill 350 enemies with any rifle. (campaign only)
Recover 12 Intel Items.
Stand united.
Hit 10 enemies with your melee attack. (campaign only)
Kick an enemy by vaulting over a cover. (campaign only)
Show mercy.
Kill an enemy just as they are throwing a grenade. (campaign only)
Buck the chain of command.
Play it loose.
Kill 50 enemies with any sniper rifle. (campaign only)
Face the horrors of war.
Kill an enemy by tagging him with a sticky grenade. (campaign only)
Kill an Edged Weapon Expert while he is up to 5 meters away. (campaign only)
Engineer an exit strategy.
Kill 3 enemies with a single grenade. (campaign only)
Kill 5 enemies using blind fire. (campaign only)
Sprint into cover 10 times while under fire. (campaign only)
We have contact.
Look out below.
Recover one Intel Item.
What's lost is found.
Play it smart.
Follow your orders.
Save a bullet.
Choose vengeance.
Complete game on "Suicide Mission" difficulty.
Kill 100 enemies with any pistol or SMG. (campaign only)
Kill 75 enemies with any shotgun. (campaign only)
Kill 50 enemies with grenades. (campaign only)
Complete any chapter with 60%+ accuracy without dying or reloading a checkpoint.
Complete three chapters in a row without being killed or reloading a checkpoint.
Kill 4 enemies with a shotgun in 10 seconds or less. (campaign only)
Kill 50 enemies using only the Attack Command. (campaign only)
Recover all Intel Items.
Complete game on "FUBAR" difficulty.
Kill 150 enemies with any heavy weapon. (campaign only)
Kill an oryx. (campaign only)
Kill 10 enemies while they use zip lines or are rappelling. (campaign only)
Stun an enemy by dumping sand on their head. (campaign only)
You are relieved.
Live and let die.
Blow up 10 explosive objects, killing at least one enemy each time. (campaign only)