Gym Leaders:
Hyouta (Rock): Turtwig, Piplup
Natane (Grass): Chimchar/Monferno
Sumomo (Fighting): Monferno (for Lucario), Prinplup (peck for the rest, I think monkey is best bet)
Maxi (Water): Grotle (except for Gyarados, but can be toppled by any Electric really)
Melissa (Ghost): Not to sure about this one.
Tougan (Steel): Infernape (Steel Killa')
Suzuna (Ice): Infernape (Ice Killa'), Empoleon (Drill Peck for Medicham)
Denji (Electric): Torterra (Can take out all but Ambipom, but Ambipom shouldn't be that hard to take on)
Elite 5:
Ryou (Bug): Infernape (except for Drapion, but he's 1x for fighting)
Kikuno (Ground): Torterra
Ooba (Fire, for only 5 fire types pre-Elite 4 ready for capture...): Empoleon, Infernape (for Steelix & Lopunny)
Goyou (Psychic): Torterra (for Crunch)
Shirona (Champ): Torterra (Milotic & Gastrodon, possibly Garchomp), Infernape (Lucario & Roserade)
That's MY belief. Torterra's good for Elite 4, Infernape good for a few Gym Battles and Elite 4, I'm personally choosing Infernape, since It's the only fire Pokemon other than Ponyta until the game is beaten...