Considering you are yet to get past castelia city, your options are limited, but there are some Pokemon that can help.
Herdier: learns the super effective dark type move bite, and is generally powerful. Can be found as a lillipup in routes one two and three
Watchog: similar to Herdier, it learns the super effective dark type move crunch. It is more powerful now but is weak in late game. Obtainable as patrat in routes 1 2 and 3 as well as the dreamyard
Purrloin/Liepard: as a dark type, it is immune to Munna's psychic type attacks. Like Watchog, liepard in powerful early on but weaker late game. Available on routes 2 and 3 and the dreamyard as purloin
Pan----: your monkey also learns bite, a supereffective move. Availble in pinwheel forest (rare) and given to you in the dreamyard.
Scraggy/Sandile/dwebble these three Pokemon hit psychic supereffectively with their dark and bug moves. Found in route four
Sewaddle & evolutions: have supereffective bug type moves. Found in pinwheel forest as sewaddle
These Pokemon may help you defeat munna. Hope this helped.