I would use Lucario. It is Fighting/Steel, so it’s typing only overlaps with Chesnaught, which shouldn’t be too much of a problem. You have a lot of Physical attackers, so I would suggest running a specially oriented set with Dark Pulse, Dragon Pulse, Aura Sphere, and Psychic. It can also Mega Evolve, and it has great stats. It can easily take down Wikstrom, Wolfric, and Olympia.
Riolu can be found on Route 22 (Détourner Way) with a 5% spawn rate at level 6 in the grass, and 25-26 in yellow flowers (source). An alternative method is obviously getting one from Korrina.
Hope this helps! :)