I really need help getting past the Hearthome Gym...this is actually my..

Question for Pokemon Platinum

I really need help getting past the Hearthome Gym...this is actually my second time playing Platinum. The first time, I didn't finish the game but (for some reason) decided to restart the experience. The first time, I think I got past the Hearthome Gym pretty quickly--I don't remember how...but I did. :P

Now though, I've battled and lost to Fantina at least 10 times. I think it might have to do with the fact that last time, I had much stronger and useful Pokemon. My current team includes:

Monferno lvl31
Ponyta lvl23
Machop lvl19
Staravia lvl14
Shellos lvl15
Togepi lvl14

None of my Pokemon know bite, which apparently is very useful against Fantina's Pokemon. My strategy's been bulking up on super potions and sending Ponyta out first with about 2 flame wheels to finish off her Duskull. Then I *try* to do the same against her Haunter, but Ponyta always gets knocked out then. So I replace him with my Monferno, who finishes off Haunter, and gets finished off by her Mismagius (though not without inflicting some damage--at least half of Mismagius's HP)

I've battled as many trainers as I could find, even going all the way back to Eterna Forest. But I only managed to get my Monferno from 29 to 31...

I need some huge help. In fact, any help would be appreciated!