How to DM a performance duel that involves the full party and has meaningful decision making

Question

The situation:

The last session ended with the party's bard on a stage, exchanging insults with an NPC bard. The NPC bard has something the party wants, and the party has something this NPC wants. The party is expecting the next sessions to start with a "rap battle", and I want to give them one. But I am unsure how to deliver it in a satisfying way. Most likely, I will have this NPC formally challenge the players, so I can use the NPC as a vehicle to set the in-universe expectations.

My goals:

1. Let the players fulfill the fantasy of having a substantial performance duel.
2. Involve the other players, who are not bards, in some way that is meaningful to the outcome so that they don't feel left out.
3. Provide rising and falling tension + pacing, and some meaningful decision making, similar to a real combat encounter. Which means that I can't just have a series of opposed Performance checks and see who's rolls are bigger.

There is a crowd, and I suspect that the crowd's reactions will determine the winner in some way.

Answer 1

I've run a game in which a turning point in the storyline was represented by a one on one boxing brawl between the party's Fighter and the local crime lord's champion (who would have won a straightforward one on one brawl).

I actually just left it to the party to come up with ways to influence this fight, which ended up being mostly underhanded. They thought of some really interesting stuff! Including a lot of distractions, subtle buffs and debuffs, stuff I could never have thought of in a hundred years of GM planning. Bribing the guards, spiking the refreshments, using illusions to summon orcish cheerleaders... Basically the party will come up with ways of using their characters to influence this showdown, all you have to do is be flexible, throw out the odd suggestion, and be ready to explain the possible consequences of failure. The deciding rolls should come down to the bard though - he is the one on the stage.

The technique you'll probably end up using will be a cumulative series of opposed checks, building to an total end goal of X successes. These checks can and should be influenced by all manner of things, and be ready to think of a few for your NPCs to use to keep it an interesting and a closely-fought thing (he busts out an amazing flute solo! advantage on his next roll!).

Use the reactions of the crowd to keep the party engaged and the tension high - do they cheer and stamp their feet, or start throwing rotten vegetables at the loser? Have a nefarious bunch bet money on the other side, and are now making death threats? Do the town guards rush over to try to break up the fight? Or one of the participant's family members?

But basically just be flexible, and let the party run riot - they will be the ones to make this a session to remember.

Answer 2

Circling back around: Timje's answer above was helpful in orienting me towards a solution. I eventually decided to go with a 4th-edition style Skill Challenge: The NPC bard will challenge the players to a duel, judged by some of the townsfolk NPCs, the following day. That will give the players an in-game evening to plan out their strategy, including any scouting or influencing they want to do on the judges.

The actual event will be handled as a series of skill checks, where the party needs to accumulate enough successes to reach a pre-defined threshold, while avoiding failures.

Last but not least, I've got a number of possible wrenches to throw into the mix based on how it's going and which party members need help finding ways to contribute. For example: Someone's dog gets spooked by the events on stage, a bee's nest is under the stage, drunkard in the crowd starts throwing tomatoes, etc.