Is my Pokemon Diamond team ready for the Elite Four?

awesomness. But thing is, you're not technically ready yet. you can beat the rest of them except for Cynthia 'cause all of her pokemon are at lv.70-77 so yeah, try to level them up a little more. But other than that, you're team is pretty balanced. Keep Luxray and just teach either Gyrados or Garchomp ice beam. Works well against Cynthia. Plus you may want to equip a Focus sash or band to one of them as Cynthias Garchomp does know Dragon Rush.