I've had quite some successes with Meganium with Heal Pulse/Protect/Light Screen/Reflect - just make sure you use Protect if there's a Fire Type against you. Meganium is best combined with a Pokemon that has moves like Earth Power, Scald or anything else that deals a heavy blow or takes out Fire types.
If you're sure Meganium doesn't get chopped up on the first turn you can skip Protect and take Aromatherapy/Giga Drain, not many people expect a 100% support/defensive Meganium. Just look at what the opponement sent in and use Light Screen/Reflect accordingly, followed with Heal Pulse/Aromatherapy while keeping up LS/Reflect.
Rash is a good nature because it boost Poliwhirl's sp attack, but it lower's his sp defence. The natures I've seen on Politoeds (I don't see poliwraths too often but I know he's phys attacker). but politoed is a sp attacker. Because of the nature, you're better off with a Politoed