Meganium's movepool is also extremely strong in-game. The only types that resist both Grass and Fairy are Fire, Bug, Poison, and Steel. Of the ones available in this game, the only Pokemon that resist both Grass/Fairy are either (a) weak to Fire, Ground or Rock. There are 6 Bug/Poisons (Weedle and Scolipede lines), 9 Grass/Poisons (Bulbasaur, Bellsprout, and Budew lines), 2 Poisons (Trubbish line), 2 Fire (Charmander/Charmeleon), 3 Fire/Flying (Fletchinder/Talonflame and Charizard), 2 Fire/Normal (Litleo Line), 1 Steel/Flying (Skarmory), and 1 Steel/Bug (Scizor).
Fletchling and Bunnelby are available in Zone 1, and as soon as you reach rank V (i.e. before the first Rogue Mega) you can get a Charmander (and Charizard's megastones are purchasable from the beginning). Charmander's line can use the TMs for Dig and Rock Tomb until Earthquake and Rock slide become available (Numel/Camerupt is also available early--Zone 8), and Meganium can learn Earthquake/Power, and has insane bulk with Giga Drain. Really the only thing you need to be scared of with Meganium + Charmander is Gyarados, but you can pick up Mareep in Zone.