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.
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.
Tate & Liza- 36-40
Hope I could, help if you want me to add Elite Four I can do that.