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