The rest of my team is Sceptile, Manectric/Magneton, Sharpedo/Crawdawnt and Swellow. Now I want a Fire-type!!!
The amount of Fire types is limited in Ruby and Sapphire, but Numel can be caught fairly early at Route 112. Because in-game is mainly focused on power and offense, I think that Camerupt will do well as its Attack and Special Attack are decent. It also provides a nice Electric immunity, which your team will benefit from because both Swellow and Sharpedo are weak to it.
Why not Ninetails? Can be caught at Mt.Pyre. Good moveset: -Will-O-Wisp -Toxic -Overheat/Flamethrower -Confuse Ray Main aim here is to inflict status on the opponent and switch out to last and inflict status on everything. This set also helps out with Steven (champion).