here s the place for grinding before Crasher Wake
I'd say the Seven Stars Restaurant is a good place, since it has daily respawning trainers and you can train up multiple Pokémon at a time, since they're all Double Battles.
Seven Stars Restaurant is located in Hotel Grand Lake
the location of Hotel Grand Lake:
Hotel Grand Lake is technically either part of Route 213 or Valor Lakefront.
Hotel Grand Lake (picture from bubapedia)
Seven Stars Restaurant (picture from bulbapedia)
Source and Source and Source and Source
Hope I helps!