You can rematch them at the the Pokemon league whose levels are the same.
You can also battles them at the battle chateau
Levels for elite four are 55 without a writ, 60 with a writ of challenge, 65 with a Red writ challenge, and 75 with a black writ challenge.
Wiksttrom: Scizor, Probopass, and Aegislash
Malva: Pyroar, Talonflame, and Chandelure
Drasna: Dragalge, Noivern, and Altaria
Siebold: starmie, Clawitizer, and barbarcle
The champions levels are 60 without a writ, 65 with a writ of challenge, 70 with a red writ challenge, and 80 with a black writ challenge
Diantha: goodra, Hawlucha, Gourgeist and gradevoir