You need a Pokemon with that ability, so you need to catch an Infernape that has Iron Fist. In this case, you can only get an Infernape with Iron Fist from the Dream World, so catch one from there.
From Generation V, female Pokémon with Hidden Abilities have a 60% chance of passing on the Hidden Ability to their offspring when bred with a male Pokémon from the same Egg group. -Bulbapedia
So here is what you need to do:
- Catch a female Infernape with the ability Iron Fist from the Dream World.
- Breed with any male Pokemon from the Field or Human-Like egg group.
- There will be a 60% chance that the baby Chimchar will have to Hidden Ability Iron Fist.
Hope THis Helped :D