i beated the game
here is my tips, hope it's useful.
start the game with Earth,
this element has weak damge but area attack
that will be useful in later battle.
then lightning,
the second Earth skills Boulder Fall can stun enemies and lightning also stun them. this strategy can kill any boss by just make them can't move.
the easter strike is the 4th skills i develop.
5th is Earthquake
6th is Zeus Wrath
I finished the game with almost 90% S rank and left 20k gems at last
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 "Wizard Defense"){kind=icon}