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