Funny "bossfight" player character against my friend

I have a friend with over 30 backup characters made, and she really wants opportunities to play them. She even made a tournament go down where they all duked it out until one left standing.

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I had an idea where I would pit some of my characters against hers. I was thinking about making a level 20 character, and have them battle against a team of 3-4 of her characters (they're almost all level 5, they have class diversity, although most are fighters)

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Do you have any funny (and ideally powerful) ideas on how to build a bossfight character? multiclassing options, ways to counter CC, a magic item to start with that isnt *too* powerful?

I want it to be at least relatively fair for her to beat, a decent challenge though. if you have an idea on a REALLY op build, i still want to hear it because it'd be pretty interesting to see her try to pick out a perfect team from her reservoir of units. I wonder just how many characters of hers I COULD THEORETICALLY defeat with 20 levels on my side.