If you are so inclined to use Munchlax, I say you should consider Little Cup battles. It's a new way to battle and shows off your skills to use unevolved Pokémon.
Here's a good set for Little Cup (according to Smogon):
Munchlax @ Oran Berry
76 HP / 156 Atk / 36 Def / 236 SpD
Careful / Thick Fat
- Fire Punch
If you want to know more about Little Cup, here's the link to Smogon's guide:
I hope this helped!