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