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