if you keep repeating level 16 and deliberately lose, after killing the first wave with a meteor, and not placing any towers, you can easily collect enough experience to max out all of the upgrades you need to complete the rest of the game with little/no difficulty, including getting brilliant on all levels. (i'm sure there are a couple of levels this trick works on, but 16 was the one I noticed, and it gives LOADS of XP, usually a rank a time.)
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