The absolute fastest way to Lvl 99

[BThumme]

Quick battle, search for c5 Siegfried name Ozanius

Pick nightmare

Go all the way left, Ozanius will chase you. Press b+g and you'll throw him out. He never blocks the grab, and nightmare has a long reach. One you get in a groove you will be on fire.
Average ring out is with :55 left. That's about 140 xp/ 30 seconds. No way you can beat this.

No complicated combos here.

EDIT: I've beaten him 63 times in one hour. 138 point/ win. That's 8694 xp/hr.