I don't like when the games require you to buy K+ cash, 4/5 for that. Look at Learn to Fly, every single thing (including special store) is free in the game.
Just a quick tip for level 9 - Stormcloud Temple.
At the bottom left corner there is a sunray tower which can 1-hit some of the opponents, hope this helps.
Amazing game, best game i've ever played on Kongregate, challenging,fun,good time killer, and UPGRADES.. what more can you want from a game? Looking forward to a sequel as good as this one. :)