I whatched walkthroughs and detonados (Brazilian walkthrough) and before the Elite 4 they have Lv.60+ Pokémon or from pokemon other Generation, while I have Lv.40+ Pokémon from Unova
The reason they have those pokemon are because A) They Cheated to get non-unovan pokemon B) they traded from a complete pokemon game. i suggest you battle wild pokemon on victory road or near it. Or maybe the Stadiums in Nimbasa City. Once a day.
Because they train and train and train until the E4.
Other Region Pokemon likely were traded from other Black/White.
for Example I Trained so much on Route 7 That I faced the League with a level 83 Samurott. (I AM NOT LYING)