For those going for Badge of the Day: Don't worry about your score; you merely have to complete the last level, so refill your health when you think you need it. I generally refilled when I got to about 30% health remaining. Also, the green enemies that explode are huge in later levels; they can wipe out whole cadres of other enemies in one go if you allow them to explode.
I feel like this game would be a lot better if both you and your opponent each had an individual grid of gems to use instead of sharing the same one. A lot of the strategy here revolves around not letting your opponent get good matches instead of trying to line up good matches for yourself. You could even add in a power-up that disrupts your opponent's grid as well. Just a thought. :)
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