Yea, Victory Road is probably the best place. Give whatever Pokemon is in the lead the Lucky Egg, and make sure the EXP. Share is turned on. It shouldn't take that long for your Pokemon to go up a few levels.
As for how to beat the champion - did you fail against her Gardevoir? If so, then you probably just need someone to teach Poison, Ghost, or Steel moves to. Even as a Mega-Gardevoir I found it to be quite frail, and unable to take super effective hits very well. I mean my level 53 Aerodactly took it out with 2 Iron Heads, and that's over 10 levels below what the Gardevoir is (admittedly I got the flinch from the first one, so it couldn't even attack me). But the Mega Evolution's Defence stat is the same as Gardevoir's regular one, so if you have Iron Head, Poison Jab, etc. on anyone, then that is definitely the way to go.
