i play this game on facebook, and for people saying it's pay pay pay, it's not. there's a feature where you upgrade your armor but it unlocks after passing certain stages of the game, where you can be end game and not have to pay.
after maxing critical hit and getting attack over 250, i put all skill points into defense now. easiest way to grind since with critical it's nearly one hit kill even at lvl63. also if you're going for level 100, they only level you should be grinding on is the volcano, everything else only slows down the process
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