Best way to level destruction?

After you have beaten the Dark Brotherhood quest line, buy the torture room and keep attacking the people chained up in there. Once their health is low, use healing hands and heal them.

I have level 100 in restoration and destruction.

Destruction magic needs to hit its target, and it depends on the amount of damage done. The more damage you cause, the more experience you gain. Even still, leveling it is especially slow, so you will want to take advantage of all the possible boosts before power leveling that skill:

• Mage Stone
• Lover's Comfort or Well Rested

The best way to do it is to find an essential NPC and roast them for a while. Bonus points if you use destruction in one hand and restoration in the other hand (to level both at the same time!). If you don't want to waste time training it this way, I would suggest getting the rune master perk and using lots of runes while you go dungeon exploring. Runes are fairly high damage and you can cast it ahead of time and allow your magic to regen before proceeding into battle.

Also, here are the destruction trainers, its worth thinking about just paying for the training since it takes so long:

• Wuunferth the Unliving in Windhelm (Common)
• Sybille Stentor in Solitude (Expert)
• Faralda in the College of Winterhold (Master)