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