All you really need are a lvl 60 Infernape and a lvl 60 Empoleon tbh.
Source: Beat Sinnoh E4 last night with basically just these 2.
Infernape sweeps Aaron (Bug)
Empoleon sweeps Bertha (Ground)
Empoleon sweeps Flint (Fire)
You will probably need both to beat Lucian (Psychic) but Infernape outspeeds pretty much everyone and with a Charcoal or Choice Specs and can sweep
Cynthia is a bit difficult and you will probably need some revives but the actual E4 is incredibly easy imo