Krakilin, you have to focus leveling up one character to full. Put one in an area where they will hit alot and get the most experience. and where your other defense won't steal the experience. Every time your unit has a chance to level up, do it quickly. I did it on the medium or hard 25 wave levels.
It's a really good concept, just needs a little more work. There are alot of great ideas in these comments. I think there should be an option to go back and edit maps that you have previously made. Plus more options for towers and baddies.
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