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.
Win an act 2 or 3 level using a single trap type
Kill an Ogre in a Lava Pit or an Acid Pit
Complete Act 1
Go through portals 20 times in one level
Kill 1,000 orcs
Get a 20 kill streak
Complete the Hallway using only the Arrow Wall trap, weapons, and spells
Get a 10 kill streak
Kill 10,000 orcs
Kill 30,000 orcs
Complete Act 2
Complete Act 3
Earn 5 skulls on every level on War Mage difficulty
Get a 30 kill streak
Get a 40 kill streak
Win an act 2 or 3 level without using any traps
Win a level with 1 rift point left
Get 50 kills with the rolling log in one level
Get 50 kills with explosive barrels on one level
Gib more than 200 enemies on one level
Get 20 chandelier kills in one level
Get 300 fireball kills in one level
Get 50 pounder kills in one level
Get 100 acid pot kills in one level
Complete Act 1 on Nightmare mode
Complete Act 2 on Nightmare mode
Complete Act 3 on Nightmare mode