interesting exploitable: pausing the game doesnt pause the star respawn timer, so if you are trying to collect suns, you can pause the game (the summoning menu) you can wait for the suns to respawn and then eat them and rinse repeat
This is my set up for beating level 21:
choose fireball and regen as your abilities,
max out endurance stat, put 2 points into mana regen, and dump the rest into life. load the level up with orbs as best you can, use fireball from the start, dont bother with regen untill the last wave. i used fire orbs primarily. should be fine though, high life + life regen + super endurance trumps the final wave
Alrighty: this is how i beat level 11:
Life - 2 upgrades
endurance - 3 upgrades
mana - 1 upgrade
mana recharge - 2 upgrades
fireball - 5 upgrades
regenerate - 1 upgrade
fire jewel - 4 upgrades
ice jewel - 4 upgrades
put one ice orb in front of yourself and then put the rest in front of the spawn point, save 2 orbs for the end. when the final boss comes, put them on the end part of whichever of the 2 paths he takes so they both hit him, then turn on your regenerate. have fireball power on the WHOLE match. GL
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 "Harmony Keeper"){kind=icon}
 "Avoid The Sad"){kind=icon}
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 "Orbs"){kind=icon}