Any tips?

Going from my memories. Use Mudkip lol. By the time it’s Swampert, you can arm it with both Surf and Earthquake and steamroll almost anything in your path. And Ice Beam gives you a nice counter to your biggest weakness too. I used to solo the elite 4 with Swampert. The only ones that give you trouble are the Ghost types that cause you to use double PP, so go prepared with Leppa berries or a pill that makes your PP go up.