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