To Get through 100% You Have to Beat the game, and get all upgrades and medals.

Upgrades, Can Refer to Shop and Equipment, I would be more specific on everything you should do, but Most of the medals have those 100% Completion Qualities

You need to get every medal, and they're all listed below.

All The Medals include:

Reach Level 30. Have Max HP and MP. Buy Every Spell. Collect Every Coin. Open Every Chest. Do Level 3 in 40 Sec. Do Level 7 in 50 Sec. Do Level 13 in 60 Sec. Do Level 19 in 70 Sec. Make a boss kill itself. Take 0 Damage to a boss. Kill 1000 foes. Kill a foe with spikes, then watch them die. Hit an enemy with a falling icicle. Collapse a plank of wood. Stand on a cloud. Death by Spikes. Death By Lava. Slip on Ice. Bounce on a Mushroom. Make an enemy stay in the air using attacks. Make an Arial combo by bouncing on foes and staying in the air. Do a Normal 3-Hit Combo. Get rid of all the Spells. Cast at least 1 Spell. Cast a Level 3 Spell. Finish Foe Rush on hard or epic. Finish Boss Rush on hard or epic. Finish Level 20. Click on the Credits in the Main Menu.