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