Based on Dephender's summary, Iris seems to be focusing on full-offense tactic, which may or may not be to prove Cynthia how far she has come, but as we humans say: "Power is nothing without control."
Iris' Journeys debut showcased her with a Gible and Goodra with Clair in background. Perhaps, Iris could use a Goodra, Dragonite and Haxorus to play the deck open with full-Dragon team. Again, a lesson she might learn how type variants can come clutch in such occassions.
Still, Haxorus against a Milotic and a Mega Garchomp seals the deal. Writers are not going to allow Ash to climb his way up against the current Monarch with relatively-easier opponents. He'll go through Steven, Cynthia, and Leon as World's Top 3.
Iris has nothing to lose. No proof required to win each battle how strong one became, after all. The victor will be Cynthia 99%.