Offhand, I'd say an beguiler with otherworldly, heighten spell and versatile spellcaster as feats chosen. You'll need to dip into flaws for arcane disciple: spider, but I think you get still spell from your class early enough (AFB).
Oh, and you have to get ride as a class skill, I guess.
It's a lot of work to to enter a level earlier than a wizard, to be honest.