Always use one last attacking move on a Stall, otherwise Taunters will punish you in no time. I'd say either to use a strong STAB move like Giga Drain (w/ Big Root) or Energy Ball (With Meadow Plate / Miracle Seed / Wise Glasses), or use a more versatile attack like Return for that reliable damage that is only resisted by two types.
Light Screen, Reflect, Leech Seed, Aromatherapy, etc, are all great utilities that Meganium can use (I'm a fan for Pokemon with supportive/Defensive niches), but the idea of a Pokemon like that is not to support TOO much, otherwise you become predictable, and without the element of the unknown, supporters have nothing.