It took me about a million tries but I finally made it through this challenge!.. I'll try describing what I did:
I'll just you give a few tips right now, and if you still don't make it I'll try explaining what I did.
1)You don't need more than one barracks. 2)Upgrading the meteor so that it scorches, is really usefull. 3)Try making a traingle of death, made of 3 upgraded artilleries 4)the rest of the towers should be mage towers.
The simple strategy to beating almost all of the levels in the game is to place barracks, and set the rally point to an area where it will block enemies in a nice place for you to surround them with Mages, Marksmen and Artillery. I find that doing this helped me on every level except the challenges where barracks aren't allowed.
