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