Easy way to get the hard badge: Wait until you get a ring bullet powerup. Let around 8 or so of the bats surround you, and let then have it. (Get a chain) Repeat once or twice more and you are golden. You should be able to do it all in one powerup! ^_^
For Challenge #23 (the finish with less than 20 hp one) I found it was easiest to leave one of the little flying guys for last. That way you can easily control how much health you have. Hope this helps someone! :)
3-4 Pierce Cannons all by themselves is def. the way to go. All you need is the 100 hp to stay afloat, and the cannons can take care of even the really big bubbles.
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