In Returnal and Diablo IV, bosses have a checkpoints in their health that trigger new mechanics and while in Returnal, the bosses might be lasting too long, in Diablo IV, if you come with a good build and a group, they last between 2-5 minutes, enough for you to run through your rotation 3-4 times and for the boss to show you all they have for 2-3 times. I think that's good. In Draw Steel, they plan for combat to take between 1.5 rounds on the fast side and 4 rounds on the slow side. This is enough for a boss to use most of their Malice features and 2 out of 3 if not all Villain Actions, each becoming more and more dangerous. This is exciting because you're trying to get them down as fast as you can and be smart about it but you pretty much know you'll see at least the 2nd strongest power they have if not the 1st. And it's even more exciting because you're usually there at your strongest of this adventure but at your weakest with your ability to heal and survive. So it's a total rush when you survive it, especially if your survived Villain Action 3 and/or the highest Malice ability. And all of this while continuously doing damage. You see the race between how fast you can get them down while still doing your best to dodge, avoid, and soak their strongest attacks. It's an exciting race instead of just hoping to get a hit every once in a while. Also, I think it's actually easier to balance because with "AC", it's a chance of whether you actually hit, and with d20 games, a big range of how much actual damage, while with these three games, you know the pretty narrow band of how much damage they do with each hit at each stage and you can plan for that: '1.5 rounds if they hit like a truck all the time' to '4 rounds if they roll like shit time after time'. In d20 games, if they roll like shit, they can spend 2-4 hours, most of it waiting, doing almost nothing.
A Boss should last as long as it takes to show most of why it's cool and not long enough to make it feel tedious.
Thinking about this, I think about the bosses of Returnal, Diablo IV, and Draw Steel. All of them don't have the classic "Armor Class".
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