Exp. Share in Small Court and Big Stadium. There's always a bunch of trainers with fairly easy high level Pokemon to take on.
Also you could try you hand at Morimoto and, if the game is Black 2/White 2, then Kouji Nishino as well.
Other high level trainers in Black/White:
Cheren (Once per day)
Bianca (Only in the Weekend)
Cynthia (During the Summer)
Black 2/White 2:
Alder (Floccesy Town)
N (Only once a season have to have talked to him on Friday in the Ferris Wheel)
Hugh (Once per day in old team plasma HQ)
Cynthia (During Summer again)
Colress (Plasma Frigate)
Cilan, Chili, and/or Cress (In the Striaton Cafe/Gym)
Shadow Triad (Once per season at the end of the bridge in Icirrus City)
Benga (In White Tree Hollow/Black City which also contains various high level trainers)
Depending if you have Black and/or White Biance and Cheren maybe re-battled once only through memory link.