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