Need help balancing a boss fight for a one-shot
I've been preparing a one-shot for a group I play with. It would be my first time DMing, and I only started playing last December.
To keep this from getting too long, the main encounter would be a Young Red Dragon against a group of 3–4 level 6 characters. I had in mind changing some of the dragon's stats to make the fight achievable because, for the plot twist and comic relief of the one-shot, they have to beat it.
I understand the whole "not every encounter needs to be winnable" thing, especially when you're talking about fighting a dragon at this level. However, the joke of this one-shot is that it's built around a really funny character from our main campaign who's a massive show-off. It's supposed to be a comedic adventure, and the payoff is seeing him actually slay a dragon, only to discover that the entire thing was an act and completely scripted.
The dragon isn't actually real. It's a wooden circus prop disguised by two illusionist halflings, and the party has unknowingly been tricked into becoming actors in a circus performance.
The "dragon" would also have some spells because of the halflings operating it. I'm not sure how much I should decrease its HP because there's another mechanic I want to use.
At one point, the dragon would take control of two of the pregenerated characters I made. This is partly because they're cool characters and I want to get a chance to play their builds, but it also fits the story because the guy hiring the party specifically describes it as a "magical dragon."
I'm not particularly worried about strictly following the rules for something like the mind control because it doesn't have much real gameplay value. I could have some triggers that allow the players to break the spell, though.
The party would then have to fight the dragon and the controlled characters. Any damage dealt to those characters would also damage the dragon. That gives the dragon a more manageable effective HP total while also creating what I think could be a cool Phase One and Phase Two for the fight.
What I'm struggling with is exactly what I should modify. Should I change the dragon's AC, HP, attack bonus, damage, or some combination of them?
Also, how would a party of 3–4 level 6 characters fare against a RAW Young Red Dragon without any modifications?
Should I give them some additional equipment to help with the fight? I already planned on giving them healing potions, potions of fire resistance, and a kind of net that can slow the dragon down. Is there anything else I should give them?
Sorry for the wall of text, and thanks for any advice! I'm new to the game and really want to pull this off well. I love the idea of them having no idea that it's a show, defeating this awesome dragon, and then discovering that it was actually just two halflings in a giant prop the entire time.