go up to the graybeards mountain, and go into High Hrothgar. When borrii, Arengar, or someone else is kneeling and praying in the main room, sneak and attack them. You can wait an hour for their health to regenerate. It MUST be don't with a low damage dagger. Save often in case they turn hostile. most of the time, they won't. You can get 100 in 20 minutes.

Most people have known this for 2 years already.

You could also just sneak attack either Hadvar or Ralof.

Or just sneak into a wall behind that blind guard in White River Watch, or in the corner of Dragonsreach.

Anyway, getting a skill to 100 early is a good way of getting killed when all the enemies are leveled to your level and you have almost no combat skills leveled up.