This gym has Rock-type Pokemon. Grass, Fighting, Steel and Water types are very effective against it. Having Mudkip and Sceptile as your starter puts you into a good advantage but if you select Torchic, you have to resort to another Pokemon. If you don't have access to Pokemon Bank (and transfer some low-level Grass/Fighting/Water -types from Pokemon XY), or a friend you can trade, you can try your luck in Wonder Trade or post in GTS. If you don't have connectivity, then the Makuhita you received will be your best bet. You'll have to spend time training it.
The Gym itself is pretty straightforward. You can choose to challenge the three gym trainers or avoid them by using the paths next to them. The second youngster’s Geodude has the ability Sturdy. It cannot be knocked out in one hit.
Youngster Josh (Prize Money: P144)
Youngster Tommy (Prize Money: P160)
Schoolkid Georgia(Prize Money: P200)
Roxanne's Geodude and Nosepass have the Sturdy ability, which prevents them from getting KO'd by one hit, especially by super effective moves. Her Pokémon’s signature move is Rock Tomb, a move that can damage and slow down your Pokémon. She will also use a Potion on her Pokémon if they are low in HP.
Gym Leader Roxanne (Prize Money: P1680)
After defeating Roxanne, you'll get the Stone Badge and TM39 Rock Tomb. All Pokemon up to LV20 including those in Trades will now obey you. You'll also be able to use Cut outside the battle.
After leaving the gym, you'll see a scene. You now need to head to follow the Team Aqua thief to Route 116.