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