Here is the full list of all 27 Orcs Must Die! 2 achievements. It takes around 40-50 hours to unlock all of the achievements on Windows.
Complete campaign, Act 2 Nightmare
Kill 50,000 Orcs
Complete campaign, Act 1 Nightmare
Collect 30 dropped skulls
Complete Twisted Halls in co-op
Beat a nightmare level using only weapons, trinkets, and the dwarf, archer, and paladin minions
Complete campaign, Act 2
Earn 5 (base) skulls on every campaign level on War Mage difficulty
Complete all classic levels
Kill 25,000 Orcs
Reach wave 50 in an endless mode level
Get a 10x or higher combo
Reach wave 25 in an endless mode level
Get a 25 kill streak
Catch the Robot disease
Reach wave 40 in a co-op endless mode level
Heal an ally in co-op
Use the freedom trinket to break a co-op partner out of a stun
Kill 1,000 orcs with minecarts
Spend 1000 skulls in the tome
Spend 100 skulls in the tome
Complete campaign, Act 1
Charm 100 orcs as the Sorceress
Use the berserker trinket to reduce your health below 15%
Spend 10 skulls in the tome
Respec your character
Get a 10 kill streak