try to make your maze so that a fully upgraded squirt tower (or 2 or 3) end up around the middle of the field, that'll help with the flyers. also, a fully upgraded swarm tower can almost handle the flying bosses all by itself. i always force the creeps into the top left corner by making rows of pellet towers that lead them there, then build /one/ maze that, again, has most of the firepower centralized, maximizing the use of each tower (and therefore your money). good luck :)
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