I wouldn't want him as the champion but I'd love to be able to battle him in some sort of post game side quest where you can find and battle old protagonists. It has to be a double battle and his first two Pokémon should be Espeon and Umbreon. You can also only see him during the day in the desert and when the weather is really hot (old protagonists would all have their own requirements for encountering). When you battle him, the weather is always intense sunlight by default during the battle. His team should be:
Espeon (Male) Lv85 Psychic Morning Sun Helping Hand Power Swap
Umbreon (Male) Lv86 Payback Moonlight Helping Hand Guard Swap
Meganium (Male) Lv85(if you chose Chespin*) Solar Beam Synthesis Aromatherapy Light Screen
Typhlosion (Male) Lv85(if you chose Fennekin*) Eruption Solar Beam Protect Reflect
Feraligatr (Male) Lv85(if you chose Froakie*) Aqua Tail Surf Rain Dance Roar
Entei Lv86(if you chose Chespin*) Eruption Solar Beam Roar Reflect
Raikou Lv86(if you chose Froakie*) Thunder Rain Dance Protect Reflect
Metagross Lv87 Meteor Mash Zen Headbutt Protect Light Screen
Tyranitar Lv88 Stone Edge Payback Earthquake Reflect
*Only for the first encounter. His team cycles from his Chespin, Fennekin and Froakie team in that order each time you defeat him and battle him again another day.
By the way, he tends to use his Pokémon in pairs: Espeon + Umbreon Meganium + Entei Typhlosion + Suicune Feraligatr + Raikou Metagross + Tyranitar
When using Typhlosion and Suicune, he tends to play defensively with one and offensively with the other depending on his opponents. If his opponents are weak to Fire he never uses Rain Dance, if the weather is intense sunlight he never uses Surf, if his opponents are weak to Water he uses Rain Dance and never uses Eruption and Solar Beam.