Right now they are at high forties and low fifties.
The Elite Four all have Pokemon ranging from Levels 54 - 55, and Kukui's highest is Level 58. Therefore, if you want more of a challenge, you should use Pokemon in the low fifties, possibly up to 55. It's very easy to get levels in the game, so you'll probably end up on par, if not higher than, Kukui's ace.
Hope I helped!