Oookay, seems I was just lucky the first time around ;) Don't follow my strategy if you want to live, haha.
I haven't lost yet, but i seem to lose my knight every level.
I just beat all ten levels with one single death.
I used one unupgraded knight in front of two nearly maxed mages.
Since I was stupid enough not to replay the level where I lost one, I'm gonna do it again now.
By the way, just wait out the first turn until they get into range, and try using the knight to hit other knights before you zap them, so they won't block the magic.