So many people have problem with this segment to this day, so if you are ever stuck, just do yourself a favor and get these 3 pokemon to get it over with.
It is so straightforward you don't even need to use healing items during battle, and in this example, If i had handled it differently, my Ninetales might have not even died.
I'm pretty sure there are many other combinations, like Aerodactyl, Tyranitar, and something, but this one is guaranteed to work as long as you don't miss Heat Waves.
Charizard: Super Rod in the lava of Pyrous Mountain after reborn renovation
Ninetales: A Clown in the second floor of the gateway between the Circus and Agate, will want a Stunfisk.
Beedrilite: Collectible within the Glass Factory (before the series of battles)
Example: