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.
Recover all Intel Items.
Choose vengeance.
Kill 50 enemies with grenades. (campaign only)
Kill an enemy just as they are throwing a grenade. (campaign only)
Get out of here!
Kill 100 enemies with any pistol or SMG. (campaign only)
Stand united.
Kill 250 enemies with headshots. (campaign only)
Kill 5 enemies using blind fire. (campaign only)
Kick an enemy by vaulting over a cover. (campaign only)
Save a bullet.
Kill an oryx. (campaign only)
Kill 350 enemies with any rifle. (campaign only)
Follow your orders.
Play it loose.
Kill 50 enemies using only the Attack Command. (campaign only)
Recover 12 Intel Items.
Face the horrors of war.
Kill an enemy by tagging him with a sticky grenade. (campaign only)
Kill 50 enemies with any sniper rifle. (campaign only)
Kill an Edged Weapon Expert while he is up to 5 meters away. (campaign only)
Engineer an exit strategy.
Look out below.
We have contact.
Kill 3 enemies with a single grenade. (campaign only)
What's lost is found.
Recover one Intel Item.
Play it smart.
Buck the chain of command.
Show mercy.
Choose restraint.
Complete game on "Suicide Mission" difficulty.
Kill 75 enemies with any shotgun. (campaign only)
Sprint into cover 10 times while under fire. (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)
Complete game on "FUBAR" difficulty.
Kill 150 enemies with any heavy weapon. (campaign only)
Kill 10 enemies while they use zip lines or are rappelling. (campaign only)
Hit 10 enemies with your melee attack. (campaign only)
Stun an enemy by dumping sand on their head. (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)