the difficulty is fine as is
When playing you need to be smart about where you put towers / traps and how you use the talents.
Tips:
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Scatter towers far enough so that their radius (range) don't intersect while at the same time covering as much road ground as it can. This will make it so there aren't any wasted shots on enemies that die while one tower hits and the other misses the already dead target.
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Check to see what hue the enemies are, from what I have noticed, if you attack a mob that has the same color of the tower that's attacking it, the mob will take less damage. (not entirely certain about this one)
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Don't use spare gems (gems with a hue you are not using) as gem bombs, unless it is entirely necessary (like taking out nodes, Tower Block, Trap Block, Arcane Blocks). You can save them and transmute three of them at a time by gembombing them with a color hue you need, they need to be on the same row (left to right [x|x|x]) It can help you build up higher grades with colors you want / need for the level.