we're going to have to get out of the whole one only person boss fight thing if we do more bosses. because besides the whole "oh, screw the cast of powerful player characters, I'M mister super special awesome, so there, suck it and watch me do some cool boss battle shit." vibe it lends, having people's characters watch someone fight, or get called assholes or such when interfering will really kinda slow things down. Also makes a poor read.
Way to completely miss something that's already been touched upon. It's not about telling other characters to suck it, stop being mad your character was stopped in his tracks then killed, it went down that way entirely because of IC reasons. It's about not having to resort to massive levels of unfun, asspull bullshit for the boss when he has to fight a group of characters that would otherwise steamroll it pretty damn hard. Because our characters should steamroll pretty much anything in a group encounter, and having the one thing fight some 4+ people turns into a clusterfuck of it no-selling like there's no tomorrow to try and present some manner of "challenge" that is more of an arbitrary post count, where the goal is just to reach a certain level of posts in order to beat it.
See, the right answer here is to not have just a single bossfight for all the group, even some manner of colossus would get annihilated because it would just be a giant target for a guy that can cut anything, a dude that can summon guns at will, an infinitely strong guy that can punch anything, 2 touhou characters with their super powerful magics and so on and so forth unless the GM constantly nullifies every single attack with little to no explanation to try and present the kind of "challenge" I described in the previous paragraph. The right answer is to actually have multiple encounters tailored to each player character, something to engage each of them, play to their strengths or "weaknesses", which in this RP means "things they can't do yet", and give them their own scenario.
If you want examples, Ganondorf, the sword one, could've easily remained Bee's fight because sword duel and he actually had the Master Sword, so that's one down. For Okuu and Cirno, have Twinrova turn up because they sense their master is in danger, both groups use ice and fire magics, it's tag team match time. Chaos being a pain in the ass for Ganondorf? Have him get chucked into a bossfight with either Morpha or Bongo Bongo. Ajax is getting restless and Ganondorf doesn't want to trade punches with the guy? Summon a fuckmassive Ultra Dodongo he's been breeding as an attack beast. Get the idea? Actually let everyone have their fun without having to take turns because they should demolish everything as a group? Yes, this actually takes effort from the GM, being a GM isn't easy.