Well, if you're using the demo the game doesn't really give you a chance to do much meaningful with magic, since most of the stuff is higher-level. (In fact, I'm convinced from reading the spell list that the really cool spells start coming at level 5.) I find the main key is experimentation. At the moment, I'm trying to create an army of pure dryads (Ethereal and Awe mean that few would dare hit them, or so my theory goes) using Call Wood Spirits and Gift of Reason.
As for mages that 'rain death', you'd be amazed at the destructive might of five mages casting Raise Skeletons, though that's all I really know as far as combat magic goes. Off the battlefield you can get a lot from casting Twiceborn on all of your Death Mages and sending them to suicide. That gives you Wight Mages, which can 'train' 8-10 undead units each for no cost each turn. Get five or so Wight Mages, and it adds up. Especially in the demo, mages aren't much on their own. A large chunk of mages that know Call of the Wild or Arouse Hunger can launch surprise attacks from halfway across the map. Blood Magic has some really powerful summons and really fun ways of messing with your foe, but none of that comes in until post-demo levels. Astral also has some really cool stuff, but again not in the demo. I don't really know anything about Fire, Water and Wind, (Though they seem pretty useless in comparison to the others) and only a bit about Earth. (Though they seem pretty solid)
Yes, I know, lots of 'in the demo's. I'd give you a wider range of information if I could.
