My team in Diamond is
Infernape lvl. 60 Moves: Flare Blitz, Earthquake, Rock Slide, Close combat
Roserade Lvl. 57 Moves: Toxic, Ingrain, Petal Dance, Water Sport
Crobat Lvl. 59 Moves: Fly, Poison Fang, Confuse Ray, Air cutter
Floatzel Lvl. 60 Moves: Surf, Waterfall, Ice Beam, Crunch
Luxray Lvl. 55 (yes I know, but back in gen 1-5, exp share was basically useless) Moves: Thunderbolt, Thunder Fang, Crunch, Strength
Kadabra Lvl. 53 Moves: Future sight, psychic, Trick, Miracle eye
i have fought Cynthia in Platinum and I won due to the level decreasing, but I don't know if it will be the same in diamond because I have already lost. TWICE!!! Thank you.
Btw, I tried the focus sash, trick troll on Cynthia and it got me down to my infernape and her lucario. I used flare blitz then it KOed it. I thought I won, but the recoil killed me.