For Level Grinding, there is Ace Trainer Ariana on Route 228 who has a Chansey, a Rapidash, and an Ambipom.
You can rematch her using the VS Seeker after walking 100 Steps.
For the E4 Rematches, their levels range from 65 - 78 from Aaron's weakest to Cynthia's strongest.
The 2nd rematch, Their levels range from 75 - 88.