Question

This is my only non lvl 100 pokemon in my party.

Answer 1

Train it at:

• Victory Road: Long, but worth it.
• Big Stadium: Trainers that give good experience. Levels around 65.
• Small Court: Same as Big Stadium.
• Pokemon League: Long, and jam-packed with experience.
• Cynthia: High levels, good experience.
• Game Freak: Same as Cynthia.
• Audino: Good experience for such a Pokemon. Hunt them down for some EXP.

EDIT: Added descriptions + Audino.