[Encounter Building] Boss Fight Against a Single Humanoid

TL;DR: How do I make a combat encounter against a Single Human feel climactic and challenging, yet balanced?

Think of those climactic boss fights in action games like Dark Souls. Gwyn, Artorias, Abyss Watchers (Phase 2). Or something closer to DnD: Final Fantasy, versus Sephiroth, Kefka, or Golbez.

The story here is that the party is about to face off against a legendary warrior for reasons integral to the plot. As such, the encounter will be the party against said warrior, most likely in an open boss arena. No minions, minimal lair gimmicks, as pure a fight against a single creature can be.

For reference, think of this guy as a Master of Arms. A guy that can demonstrate expertise with any form of weaponry and martial tactics. A Battlemaster's fantasy. Think of characters like Firion from Final Fantasy, Dante from Devil May Cry, Jax from League of Legends, or Lancelot from Fate.

I've already thought of a few measure to make sure it lasts more than a single round:

• Legendary Resistances & Indomitable – This should help him save against those pesky Hold Persons, Stunning Strikes, and Slows. Hypnotic Pattern I'm not as worried about since they'll have to attack him eventually, but Readied Actions can burst him after his turn so…
• Break Bars & Phases – Taking inspiration from FGO, the guy will have multiple HP bars of sorts. Bringing one bar to 0 will not carry over any excess damage. This should help his survivability. In addition, breaking a number of bars will trigger the next phase of the fight, granting him additional abilities to keep the party on their toes.
• 2 Turns per Round – In addition to Legendary Actions, having the guy go twice per round should help balance the action economy. But I would like some input as to when those Leg Actions should refresh for balance reasons. I'm leaning towards them refreshing only on his first turn but maybe a different input is better?

Now for problems I'm having:

• Slugfest – The way I'm envisioning it right now, it's looking to be a slog of a fight since it's just one guy. I could, of course, give him a few mobility options so he can threaten the backline from time to time. But what's the best way to prevent basically going, "He attacks you thrice. Okay next turn."? I want a fallback in case my brain stops working and I can't give descriptions to color everything anymore.
• AoE – Again, this is just one guy, and with a (mostly) Fighter template at that. Unlike dragons, he doesn't have a massive breath weapon to threaten the party with. What are some feasible abilities he could have to threaten multiple PCs at a time?
• Numbers – What's a good amount of HP per bar? I don't want it to be so low that it melts after a round, but not so high that they're stuck on that phase for longer than they need to be. For reference, I'm expecting a party of 6 with an average level of 8. Mostly minmaxed party with Sharpshooters and Great Weapon Masters as well.

I appreciate any feedback. If you have additional questions or clarifications, you can ask them in the comments and I'll do my best to provide additional information.