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