How do i beat cynthia:pokemon platinum?

Your pokemon are pretty useless. Teach Floatzel Ice Beam in Aqua Jet's place. Teach Infen Flamethrower. Teach Jolteon Thunderbolt and a better move in Quick Attack's place. Ok first use a pokemon you won't use on Cynthia's other pokemon to beat Spiritomb which has no weakness (eg Bastiodon or Toxicroak). Floatzel's Ice Beam beats Garchomp and maybe also Togekiss. Infernape beats Lucario and Roserade, you can also beat Roserade with Togekiss. Jolteon beats Togekiss and Milotic.