Here is a list of 5 levels harder than Tartarus :) This is my opinion, don't hate me lol

Credits :

Thumbnail : Balmeyer
   / balmeyer  

Sonic Wave Infinity :    • Sonic Wave Infinity [FULL LEVEL] [Leg...  

Shinigami :    • [CORRECT SONG] Shinigami Last Preview  

Firework :    • FIREWORK FINISHED! (UPCOMING TOP-1 DE...  

Orochi :    • OROCHI | SUPER EXTREME DEMON | Preview 2  

Galaxy Collapse :    • Galaxy Collapse (Layout) | New Hardes...