Defeating the Elite Four is pretty easy:
Sidney:Brick Break,Flamethrower and Thunderbolt will do the trick.
Pheobe:Phychic is the best for her.
Glacia:Thunder or Thunderbolt and Brick Break.
Drake:In comes Ice Beam!
Wallace:Solar Beam and Thunder or Thunderbolt witha high level will do the trick.
No need of High level Legendaries that way...