Actually, the dreamyard trainers are pretty high level.
But if your Pokemon are truly strong, and you know how to fight, you can beat them! You'll just have to heal a lot and take a lot of hits.
(Also, if you want to catch Musharna, show up on Fridays!)
And l do NOT suggest you battle the Elite Four again...
they will be Lv 75s or so.
l think you should keep going along the path past Opelucid and battle the wild Pokemon in the grass, they're pretty high level!