Here is the full list of all 18 Costume Quest 2 achievements. It takes around 12-15 hours to unlock all of the achievements on Windows.
Used the Pterodactyl Costume to flap 10 flappy things
Collected over 4 thousand candies
Upgraded 3 costumes
Tried on 5 different costumes
Defeated Kronoculus and his Kronys
Collected all Creepy Treat Cards
Used the clown horn 1000 times
Bashed 10 Monster Patrols
Defeated Ophirion
Found the Wolfman
Found the Solar System
Got the liberty kids to bust a move
Completed all quests in the game
Used every costume special in combat
Tried every Creepy Treat Card in combat
Restored the timeline to save Halloween
Mastered all combat techniques from Corvus
Finished the game having fought every battle with Candy Corn in your party