Ok, I'm going to average out the levels of the gym leader's mons, then round it, and add one.
Roxanne's Pokemon have the average level of 14.5, or 15. With my one level addition, it is 16. Thus, level 16 seems good for Roxanne.
Brawly's Pokemon have the average of 17.5, or 18. With my one level addition, I would say 19 is good.
Wattson's Pokemon have the average of 21.7, or 22. With my addition, 23.
Flannery's Pokemon have the average of 26.7, or 27. With my addition, 28.
Norman's Pokemon have the average of 29.7, or 30. With my addition, 31.
Winona's Pokemon have the average of 31.5, or 32. With my addition, 33.
Tate and liza's Pokemon have the average of 42. I would actually say 44 because you probably aren't used to double battles.
Wallace's poekmon have the average of 41.4, or 41. I would say 45, because I suggested 44 and you can't go back in levels lol.
Now, the elite four. I'm going to average all of the elite four Pokemon, and give a rough estimate on what I think your Pokemon should be for the whole battle.
Side note: The "algorithm" I'm using is making it more challenging by only adding one, if you want to add more, go for it.
I got the average of 50.4, or 50. However, they are the elite four, and with healing stuff, I would add 3 levels, so 53.
Finally, Steven. He has an average of 56.3, so I would recommend 59 (he's the champion)
Tate and Liza: 44
Elite four: 53
My sources were the respective bulbapedia pages.