Well, the Lucky Egg helps in one way or another. It always helped me with my Lv. 60 Black 2 team, so with a Level 90 Pokemon I'm sure could work just as well. xD
Audino can also be a very experience-consuming Pokemon. Be sure to find some, especially at high levels like near or at Victory Road. Also, you can do the Giant Chasm. With the Lucky Egg applied, you'll be counting stars. :D
You can also try your luck at the Big Stadium and the Small Court - trainers are there every time and there's a good damn lot of them, with high-levelled Pokemon.
And, of course, the strongest trainers in the game are Sinnoh Champion Cynthia in Undella Town, and Game Freak Morimoto in Castelia City.
So, there you go.
Hope I helped. :)
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