Here’s a clip I made that shows how to level destruction in Skyrim.
Note that I have the Mage Stone buff in this video.
What you need:
• Fire Rune
• Firebolt
• A follower (Lydia, in this case)
• Access to High Hrothgar and one of the Greybeards
How it works:
• Cast Fire Rune on the floor. Hit it with Firebolt. Make sure that both your follower and the Greybeard are in the blast radius.
• Profit!
• Be careful. Blast them too often and the Greybeard will go hostile and/or the follower will quit/die.