Rain Dance on the first turn, then sweep with the boosted speed from Swift Swim. Thunder never misses in the rain, Surf gets a 1.5x boost in power, and Ice Beam hurts grass types. If you have another touhoumon to use Rain Dance, drop it from Nitori and consider Thunderbolt, which you can use when the rain ends or if someone like Koishi (her ability blocks weather effects) comes in. Counter and Mirror Coat are also viable, but the former is an egg move (so you can't use it before the Elite Four if you just want to play ingame) and the latter may not be viable since Nitori has bad special defense.
