Here is the full list of all 25 Jet Set Knights achievements.
Find 1000 Coins
Slay the Muffin Boss
Destroy the Robot Boss
Shot 500 Ice Arrows
Slay the Snowman Boss
Shot 500 Arrows
Slay the Big Ape Boss
Find the secret character.
Slay the Octopus Boss
Find the Legendary Monkey!
Unlock the Owl Artifact
Unlock All Artifacts
Find 500 Diamonds
Shot 500 Fire Arrows
Use 500x Voodoo Doll
Die 100 Times
Build 100x Guard Tower
Slay 5000 Monsters with Blade
Defeat 10.000 Monsters
Slay 100 Devils
Unlock All Minigames
Use Artifact-o-Matic Machine 100 Times
Destroy 25 Training Dummies in Bonus Stages
Destroy 420 frozen monsters
Use Dash 100 times