He's got a warrior-monk aesthetic to him. Giving him mons to fit that theme is tough but would be pretty cool.

His ace is a Grass/Ground ultra beast with high Def, SpAtk, and Speed. It's at the end of his team and will (probably) be the last mon he sends out.
I'm thinking Mega Gyarados as his fifth mon maybe. A bulky water type that can still blow through a team.
Could probably give him a Fire-type for the generic Fire/Water/Grass core.
Bronzong fits the monk aesthetic, but it's kind of a shitty mon in single player. It wouldn't give much of a challenge.
Maybe a Minior or something since ultra beasts have that cosmic entity thing going on.

As the player, you're pushing Lv100 at this point and have access to most mons in the game, probably including a few legendaries of your own. This boss fight is an optional postgame thing and while it's not necessarily the final boss to end all final bosses, I do want it to be a challenge worthy of the content he's capping off.