The starter, Squirtle is beneficial for this gym due to its Water-type attacks. Geodude and Diglett can use Ground-type attacks like Earthquake to defeat those Pokémon quickly. You could also catch some Water-type Pokémon like Tentacool, Gyarados, Horsea etc. by surfing and fishing on previous water routes to help out, but make sure that they do not have a secondary type as Ice.
The gym leader Blaine will give you the Volcano Badge after his defeat, which allows you to access a brand new region, the Sevii Islands archipelago. This set of islands may not be mandatory for you to progress further into the game, but they can be a good training ground to level up your Pokémon before the final gym.