I would definitely start grinding your Pokémon to at least Level 29 or 30. Luckily, you have 3 Pokémon with a type advantage over one of her Pokémon, so you don't need to grind as much.
Maylene's Pokémon are a Meditite at Level 28, a Level 29 Machoke, and a level 32 Lucario. Being Level 29 or 30 keeps you about On par with her Pokémon, so beating her will be decently easy with the type advantage.
Hope this helps!