I think it would be rare for a high level mage to waste spells fighting first level warriors. It just doesn't serve much purpose. The first level warriors don't pose that big a threat. Instead, the spellcasters would save their spells and search out the tough guys on the opposing side. You don't see Gandalf fireballing orcs. Sure, he'd wipe out a few, but there would still be thousands left! Not efficient use of your spells, especially when the orcs don't pose much of a threat to him. (Plus, he's got whirlwind attack.) But does Gandalf magic up the balrog? You bet!
Compared to modern times, it would be like using stealth bombers to take out primitive tribesmen armed with spears. It's a waste of valuable resources. Just doesn't happen.