For trainer experience, try:
Fisherman Cole or Fisherman Alec on Route 222 - Each one has two Gyarados over level 40, which are really easy with Thunderbolt but give great experience. Cole also has a Remoraid.
Swimmer Sheltin on Route 213 - This guy has three Gyarados, but they're in their low 30s. If you don't have a Pokemon with Thunderbolt, he might be easier, but pack Repels since he's out in the ocean.
PKMN Breeders Albert and Jennifer on Route 209 - They each have a team of four, and even before the Elite 4 I'm pretty sure most of them will be evolved Pokemon. Their final teams are Roserade/Raichu/Sudowoodo/Umbreon and Roserade/Clefable/Mr. Mime/Espeon, but I'm guessing that before the Elite 4 one or two Pokemon per team will be an evolution step down.
Fisherman Cole or Fisherman Alec on Route 222 - Each one has two Gyarados over level 40, which are really easy with Thunderbolt but give great experience. Cole also has a Remoraid.
Swimmer Sheltin on Route 213 - This guy has three Gyarados, but they're in their low 30s. If you don't have a Pokemon with Thunderbolt, he might be easier, but pack Repels since he's out in the ocean.
PKMN Breeders Albert and Jennifer on Route 209 - They each have a team of four, and even before the Elite 4 I'm pretty sure most of them will be evolved Pokemon. Their final teams are Roserade/Raichu/Sudowoodo/Umbreon and Roserade/Clefable/Mr. Mime/Espeon, but I'm guessing that before the Elite 4 one or two Pokemon per team will be an evolution step down.