Hi guys,
I'll start by saying that I'm not posting this to ask for basic balancing tips, as I've that down already. I can create encounters that are mathematically aimed towards a specific difficulty level, as long as they are creative but not insanely complex. Big action oriented guy, minions, multiple medium guys, spicy terrain, simple lair actions, sure.
Now, I love actual plays (D20 veteran), but how dahell do DMs like Brennan and Matt balance their insanely creative final bosses? This fights have VERY swingy terrain features that can straight up murder you, powerful lair actions, powerful spellcasters (a personal bane to balance i have no idea) AND change something basically every turn.
I really wouldn't know where to start to put down the actual numbers for anything here, there are so many variables opposed to a couple damage variables you can average to make a very educated guess on the actual difficulty it will pose.
These fights come up very swingy thanks to all these impediments, without really giving you the chance to average down any expected DPS by the party, and that's just to decide the enemies' HP...
How do you put the numbers down on sheets here?
My big thing is to avoid hard numbers wherever possible. EG, you've got a big spooky encounter with complicated abilities, passive/lair effects, etc etc; it's health pool is whatever the plot demands. :P Realize you underestimated the raw dps of your players? Bump that health up until it has a chance to show off what it can do and feel like a threat. Realize this thing will just murder the hell out of them? Tune it down until they only need 1-2 good hits to bring it down before they're out. Similar with saving throws, bonuses to hit, ac, etc. I usually don't commit to anything until at least a round or two has passed. Much easier to balance the interplay of complicated features and abilities after you've seen what they can do, and there's no reason you can't do that on the fly, assuming your players trust you to want them to have the best experience possible.