Your levels should always be around the same as theirs, maybe a little lower or higher. Sometimes like with Norman there are large level jumps.
Roxanne- 11-14
Brawly- 15-17
Wattson- 20-22
Flannery- 25-27
You fight Norman immediately after Flannery with no new routes in between. You will probably want to grind levels a bit in between. 28-32 depending how much you want to grind.
Winona- 31-34
Tate & Liza- 36-40
Wallace- 38-42
Hope I could, help if you want me to add Elite Four I can do that.