Here is the full list of all 27 Orcs Must Die! achievements. It takes around 25-30 hours to unlock all of the achievements on Windows.
Complete Act 2 on Nightmare mode
Complete Act 1 on Nightmare mode
Get 300 fireball kills in one level
Win an act 2 or 3 level without using any traps
Earn 5 skulls on every level on War Mage difficulty
Kill 30,000 orcs
Complete Act 3
Get 50 kills with the rolling log in one level
Kill an Ogre in a Lava Pit or an Acid Pit
Complete Act 2
Get 50 pounder kills in one level
Win a level with 1 rift point left
Get 50 kills with explosive barrels on one level
Get a 40 kill streak
Get a 30 kill streak
Kill 10,000 orcs
Win an act 2 or 3 level using a single trap type
Complete Act 1
Gib more than 200 enemies on one level
Go through portals 20 times in one level
Complete the Hallway using only the Arrow Wall trap, weapons, and spells
Get a 20 kill streak
Get a 10 kill streak
Kill 1,000 orcs
Get 20 chandelier kills in one level
Get 100 acid pot kills in one level
Complete Act 3 on Nightmare mode