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