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