A key to playing this game easily is to focus heroes' skills in destroying tiles, not enemies. Get channel, magic mirror for the mage, call to arms for the cavalier, and extermination, forge for the warrior. By the sheer chance, you'll get enough skill charges to spam. I finished the game with the best rating in one go with this.
tips for beating level 14 : get adoration skill, and max up meteors(mana too), before you enter the boss, you have to save up over 1000 to make sure that he's dead. so sell all the towers on the right hand side, and leave only ones that have splash damage. Drop meteors, and quickly build 5 temples(use shift if necessary) and you can throw one more meteor, which is enough to kill the boss if you're quick
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