I've currently only beat them once on my Japanese Platinum. If I remember correctly, I think I had the team of Empoleon, Garchomp, Luxray, Staraptor, Espeon and Lopunny. All of them were at mid-50's when I started and I think ended up just above 55 at defeating Cynthia. it was the first time I'd used a Garchomp in-game and I only did so because you didn't have to wait until you got the Strength HM and the ability to use it to catch a Gible.
Needless to say, what the rest of my team may have had trouble with, Garchomp more or less reigned supreme.
My predicted team for the second Elite Four victory will be Infernape, Glaceon, Luxray, Quagsire, Gardevoir and Altaria, but I'm still a ways away from completing the English Version, as I'll be updating in the Sinnoh Journey Sticky later on.