Apart from the other answer, if you do not have a Marshstomp, i.e. if you picked Treecko, then get a Shroomish and evolve it to Breloom at level 23. At this level, it learn the all important Mach Punch, which as a Fighting type priority move, is very beneficial to your battle, as he has part-Steel type Pokemon.
This is not required if you have a Combusken with Double Kick. Or, with Marshstomp, just sweep through with Mud Shot. With Combusken and Breloom, remember to equip it with a Cheri Berry, because they will be paralyzed, and you can save a crucial turn. It is best to train above mentioned Breloom to about level 25 or 26.
Further, this might help in Gym 5 (Normal type). Also, just as a foresight, for Gym 4, if you don't have strong Water or Ground type Pokemon, please consider Marill (evolving into Azumarill) found just before, in the Fiery Path. This can be temporary.