OK here's what you do. You need to get a mana pool so you can have wizards. Once you get enough wizards you get this magic option which is just a big explosion with spalsh damage. Basically the same as the demo guys. I think they purpossly made this the only way you can win, because at like level 17, picking up and dropping guys is completely useless.
This is also the only way to win because at the end, the guys come in large groups and waves. So you need to do the explosion once, and it will kill everything. Then you charge it up again, and another wave comes.
That's all there is to the game. After that you really can't lose. I lost because for some reason half my wizards died, and I couldnt get them back in time.