If I were an elite 4 member, I would just use my favourite pokemons. This basically means "let's go psychic types"
Silvally
Claydol
Sigilyph
Alakazam
Latias
MegaMetagross
My ace is Silvally. And I would play different types of him. My strat would be to lead with him and parting shot a bit like Guzma does with his Golisopod in the games. I love the others of the team since I was a kid, especially Latias and Metagross who are my favourite pokemons besides Silvally. I would also change terrain according to his type.
Yeah I pretty much love my Silvally, since I've used him in vgc2017
I could also play steels with Aegislash, Ferrothorn, Scizor, but I would NEVER give up on Latias. If I could keep my Latias, then steel is the way.
And I would play on Murkwater Surface because I love stalling