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