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