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