All of Blaine’s Pokémon are pure Fire-types, so it’s pretty easy to hit their weak spots. First up is Magmar, which can use STAB Flamethrower, Low Kick to hurt Rock-types, and Confuse Ray to distract your Pokémon. Water-types are ideal, while Ground-types are great too.
Next up can be Arcanine, an evolved Growlithe. Blaine’s Arcanine can use Flare Blitz for a powerful STAB assault, Outrage to hit most targets for heavy neutral damage, and Crunch to bite into Psychics and Ghosts. Water and Rock-types are strongly recommended.
Rapidash is an evolved Ponyta that’s true to its name. For attacks, it also has Flare Blitz for STAB, and Quick Attack to try and sneak in an attack before you can. Rock-types are the best, but Water isn’t too shabby either. If they’re fast, a Ground-type like Dugtrio can be good.
Finally, Ninetales is an evolved Vulpox. This one can use Fire Blast for a powerful STAB-boosted hit. It also has Quick Attack like Rapidash. Similarly, Rock and Water are solid choices, to weaken the damage from Fire Blast and counter with super-effective damage.
Douse Blaine’s fiery passion and he’ll give you his Volcano Badge, as well as TM46 Fire Blast. If you missed it, Fire Blast is one of the strongest Fire-type moves for special attackers. But it’s accuracy is somewhat on the low side. If it’s reliability you want, go with Flamethrower.
When you leave the Gym, your rival will catch up to you. He’ll mention that the final Gym is the one in Viridian City. Indeed, if you want to obtain the eight and final Gym badge, you’ll need to go over there.