Dude, after like 5 levels, Knight and Spear-men were gods. Wiz, not so much, until i started using the ultra awesome combo: spam all mana on units and drop bombs in this order, Ice, Fire, Reg, Push, and Bertha. Max times should be 2 waves of this. At least that's what worked for me. good way to go for Knight is stack attack and defense, don't even worry about push, same with Spear-men. Wiz needs at least 1 push upgrade and then use defense and attack a lot. Hope this helps!
this game is absolutely great for passing the time! dude you should totally make this an app, i would buy this in a heartbeat! i love this game. i thoroughly enjoy the skills and the massive 10 chain combo. totally faving and adding this to my playlist.