>>28781015Grass/Fairy with increased defenses and the ability Flower Veil should be decent enough.
Flower veil prevents the user (if grass type) and all its grass type allies from having their stats lowered by opponents, and also protects them from status conditions. Basically you can't intimidate, snarl, sticky web, w-o-w, toxic or thunder wave this meganium. If it doesn't get it already, give it access to magic coat to bounce predicted taunts.
It would have increased SpA to make giga drain somewhat threatening, and also gain access to moonblast. Earth power by breeding to deal with fire, poison and steel if it doesn't get it already.
Other approaches to Megameganium:
>Huge SpA (and Atk) boost with a bit more speed and also serene grace or pixilate>Def and SpD wall with magic bounce>Regenerator with leech seed, synthesis, etc. 
