1. Get a vulpix from the dude in the circus (trade it for stunfisk). Soft reset until you have a vulpix with heat wave and, again, solid stats in sp. atk and speed. Make sure its ability is drought, you can use ability capsules if it isn't drought to begin with. Change your charizard's ability to solar power if it isn't.
(1.5: If you've joined Aqua gang, finish the Job Application questline to acquire Choice Specs.)
2. Evolve and EV train vulpix until it's about level 87. (Give the choice specs to ninetales.)
3. Spam heat wave with ninetales and charizard while the sun is out. If charizard's heat wave didn't finish the job, ninetales' heat wave will.
Also, level your pokemon down a couple levels. If they get to level 91 in the first two battles you're basically boned.