Seeing as NOBODY even wrote out a moveset based on Maganium's Niche, I will:
Sleep Powder (Put the opponent to sleep)
Leech Seed / Energy Ball Ancientpower (Use Leech Seed W/ Big Root for great stalling, or Energy Ball for reliable STAB. Ancientpower has low PP Capacity, but deals with all of Meganium's weaknesses)
Light Screen (One Screen)
Reflect (TWO SCREENS)
Meganium is great for its Dual-Screening prowess. I don't get why nobody even mentioned it. It can get a nuisance out of the way with Ancientpower, or cripple the opponent with Sleep Powder, and then put Dual Screens and switch safely. Meganium's niche is the best way it can be played.
EDIT: Godammit, another Gen IV mistake. No Sleep Powder for Meganium in gen IV. Oh wait, not a Gen IV mistake, just confusion with Grasswhistle, which isn't worth it at all. -_-
In that case, Energy Ball, Leech Seed, and Ancientpower ca be played around with.