To deal with the inv boss just place a low lvl towers in the maze entrances to trigger invisibility and have many Ice (with fear) and Grass (with crit) near the gems. When the boss gets near the gems he will become visible again, then trigger frenzy and he will die really quick, as they have less hp than regular bosses. Works in almost every map, with the exceptions of the really small ones. Great game by the way.
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