Well, meganium isn't usually very offensive, first of all, you need to breed a pokemon in the "Plant Egg Group" that knows Leaf storm with a female meganium, OR have a meganium (gender doesn't matter) that knows leaf storm and breed with a ditto.
For EVs, I looked through the smogon sets and found that leaf storm wasn't used, but Il give a new set a try:
@leftovers
252 HP/252 Dfc/6Spe
Bold Nature
-Aromatherapy
-Leech Seed
-Synthesis
-Leaf storm
I mixed the sets up a bit so...The point of this is switching in multiple times and supporting your team with aromatherapy and leech seed, having synthesis to heal, and since you wanted leaf storm a lot (the reason I didn't go with a standard set) I added it as a way to go out with a bang before switching, or to KO that crucial pokemon.
Just so you know, Meganium is not a offensive pokemon, and does not usually use leaf storm so I made the best of it with this set, also, meganium is NU as a little reminder, and remember that using leaf storm leaves you with -2 SpA so that is why it is before you switch or am going to die, you shouldn't use it in succession unless you are VERY close to a KO because it kills the power of the move when your in, which is why you switch.
THANKS, HOPE THIS HELPS
~TFH