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