Dragonite would clearly have the advantage over Serperior, and SHOULD beat Serperior, but if this was actually happening, we all know Serperior would win, because Iris has no business battling Alder over Trip.
AND UNTIL Iris wants to be more than a dragon master, no one has any right to argue Iris's championship title from Black 2 White 2 games as an excuse to need to battle Alder over Trip.
First off both Ash and Trip are both anime exclusive characters, so it makes no sense to favor a game character over an anime exclusive characters, UNLESS said Game Character was supposed to favored over anime exclusive character. And anime Iris, is nowhere near being close to her game counterpart. So she has no business battling Alder outside of the fun of doing it. Ash and Trip have actual reasons to be battling Alder. Iris doesn't. unless Alder magically has a Druddigon, but that wasn't going to happen, because Alder is only going to use Bouffalant.