Here is the full list of all 66 Crystal Story II achievements.
Win all Mercenary Defense events.
Defeat Seraph.
Defeat the boss of Doom Lair.
Defeat Foxer.
Complete the Secret Quest.
Defeat the boss of Giant Land.
Defeat the boss of Lao Prison.
Defeat Chimera Prime.
Defeat the boss of Sunken Ruins.
Complete all Wanted X Quests
Use "Cyclone Dance" in battle.
Catch all the rare fishes.
Complete the bestiary.
Defeat Mechaslime.
Use "Dragon's Rage" in battle.
Defeat the Card Master in a card battle.
Obtain the Number One Card.
Win 5 Class GM Card battles.
Win 5 Class S Card battles.
Win 5 Class A Card battles.
Win 5 Class B Card battles.
Win 5 Class C Card battles.
Win 5 Class D Card battles.
Complete Chapter VI.
Defeat Calloc.
Use "Death's Gate" in battle.
Use "Trigger Happy" in battle.
Defeat the "Snowy Chillers" (Hard Mode) in the Battle Arena.
Defeat the "Highlands Flyers" (Hard Mode) in the Battle Arena.
Defeat the "Swamp Stalkers" (Hard Mode) in the Battle Arena.
Defeat the "Desert Crawlers" (Hard Mode) in the Battle Arena.
Defeat the "Small Town Gang" (Hard Mode) in the Battle Arena.
Complete Chapter V.
Use "Lucky Strike" in battle.
Use "Dragon's Breath" in battle.
Use "Magic Bullet" in battle.
Use "Hurricane" in battle.
Defeat "The Big 3" in the Battle Arena.
Defeat the "Earth, Wind and Fire" in the Battle Arena.
Defeat the "Snowy Chillers" in the Battle Arena.
Complete all Wanted Quests
Get a Gold time on Track 3 of Snowboard Extreme!
Successfully hack all of the hackable chests.
Complete the Trickster tree.
Complete the Healer tree.
Complete the Thief tree.
Complete the Guardian tree.
Defeat all match defense quests.
Complete the Mage tree.
Complete the Slayer tree.
Use "Rend" in battle.
Use "Snipe" in battle.
Use "Cursed Dagger" in battle.
Defeat the "Highlands Flyers" in the Battle Arena.
Complete Chapter IV.
Use "Dragon's Roar" in battle.
Defeat the "Swamp Stalkers" in the Battle Arena.
Complete Chapter III.
Defeat the "Royalty" in the Battle Arena.
Defeat the "Desert Crawlers" in the Battle Arena.
Complete Chapter II.
Defeat "The Dead" in the Battle Arena.
Complete Chapter I.
Defeat "The Small 3" in the Battle Arena.
Defeat the "Small Town Gang" in the Battle Arena.
Win your first battle.