Inside the dojo, you’ll have to fight your through pairs of Black Belts. They specialize in Fighting-types, so you should know what Pokémon to send out. If you’re unsure, you can come back here later. Especially because you can get a powerful Psychic TM and a good Psychic-type if you keep exploring Saffron City. Note that you can fight the Black Belts one at a time if you speak to them first.
Black Belt Hitoshi
Hitoshi’s Primeape only has Normal and Fighting-type moves. An alternative to using Psychic and Flying-types is using Ghost-types since they’re immune to all of its moves.
Black Belt Aaron
Aaron prefers his Rock-types, so switch to a Water or Grass-type if possible. This Graveler has powerful Rock and Ground STAB moves, so be mindful.
Black Belt Hideki
Hideki has a Water-type that may become a Fighting-type one day. Try to lead with a Grass or Electric-type. If you grabbed it from the Saffron City Pokémon Center, Alolan Raichu is a good pick.
Black Belt Mike
Finally, a Fighting-type again. This brute only has Fighting-type moves, so again, Ghost-types are worth considering.
After going through all four Black Belts, you can battle the Karate Master waiting at the end.
Karate Master Koichi
This is a fully evolved Poliwag that has gained the Fighting-type. As such, it’s weak to Grass, Electric, Flying, Psychic and Fairy. You should have one of those for sure. A partner Pikachu can take it out in record time, while a partner Eevee can risk a Buzzy Buzz or Glitzy Glow.
Also, in the Fighting Dojo, there’s a Coach Trainer hiding near the bottom-right corner. Speak to her to begin a quick challenge separate from the rest of the dojo.
Coach Trainer Leona
Both of Leona’s Pokémon are packing an electrifying Thunder Punch. So you’ll likely want to keep your Flying and Water-types away. Against Machoke, a Psychic-type would be great. Meanwhile, a Ground-type will make short work of Electabuzz.
Fend off Leona’s assault and you’ll be rewarded with your own personal, signed copy of TM23 Thunder Punch. That first part might be slightly inaccurate though.