Normal is easy once you get a good maze set up. Getting the higher scores is all about sending the next waves as fast as you can handle them. I just did a quick runthough and got a score of 6575, but I got a little carried away at the end and sent a couple waves a little too quickly. I ended up letting one of the flyers on level 49 get past me, but that's all.
Hard is a different problem, cause my normal map doesn't really work there. I've been tinkering with a couple options, one them being a semi-dirty trick, but I haven't gotten the maze or the timing quite right on it yet.
Personally, I prefer this game...http://www.candystand.com/play.do?id=18047