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.
Complete game on "Walk on the Beach" difficulty.
Complete game on "Combat Op" difficulty.
You are relieved.
The end of the line.
Kill 50 enemies with grenades. (campaign only)
Choose vengeance.
Kill 75 enemies with any shotgun. (campaign only)
Recover 12 Intel Items.
Kill 250 enemies with headshots. (campaign only)
Get out of here!
Kill an enemy just as they are throwing a grenade. (campaign only)
Kill 50 enemies using only the Attack Command. (campaign only)
Stand united.
Kill 4 enemies with a shotgun in 10 seconds or less. (campaign only)
Save a bullet.
Sprint into cover 10 times while under fire. (campaign only)
Kill an enemy by tagging him with a sticky grenade. (campaign only)
Kill 100 enemies with any pistol or SMG. (campaign only)
Hit 10 enemies with your melee attack. (campaign only)
Follow your orders.
Kill 50 enemies with any sniper rifle. (campaign only)
Play it loose.
Face the horrors of war.
Kill an Edged Weapon Expert while he is up to 5 meters away. (campaign only)
Engineer an exit strategy.
We have contact.
Look out below.
Kill 5 enemies using blind fire. (campaign only)
Kill 3 enemies with a single grenade. (campaign only)
Recover one Intel Item.
What's lost is found.
Play it smart.
Buck the chain of command.
Show mercy.
Choose restraint.
Complete game on "Suicide Mission" difficulty.
Kill 350 enemies with any rifle. (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.
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)
Kick an enemy by vaulting over a cover. (campaign only)
Carry on, soldier.
Live and let live.
Live and let die.
Blow up 10 explosive objects, killing at least one enemy each time. (campaign only)