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