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?
Start by not really worrying about balance too much. CR is next to useless, even at very low levels. Just feel it out experimentally over a few battles, and if you go too big, withhold some of the nastier enemy abilities.
I do those big creative boss battles. I start by finding the most similar enemies to my custom enemies and running some basic numbers in Kobold Fight Club (maybe 5 minutes). Then I eyeball a rough difficulty multiplier based on their level, classes, and magic items.
My players are currently level 17 with minimal magic items and based on some experimental results I take whatever Kobold gives me for extremely deadly and then multiply the difficulty by about three to actually have a difficult battle. That might mean more enemies, or it might mean more damage, higher HP, higher AC, increased saves, etc. Right now a challenging battle for them is one in which I take a put maybe three CR 20-something monsters on the field against my party of five.
Yup, always go higher than the book says, especially with a high-level party. Tier 4 PCs have so much stuff they can do it's really hard to challenge them, so you gotta get nuts.