Mega Meganium
Grass
HP 80 --> 80
Atk 82 --> 107 (+25)
Def 100 --> 125 (+25)
SpAtk 83 --> 108 (+25)
SpDef 100 --> 125 (+25)
Spe 80 --> 80

Ability: Full Bloom -
While this Pokemon is not under a status condition, its Atk and SpAtk are increased 2 stages.

That should do it.