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?
Well, somewhat. With the way healing works in 5E it's pretty easy to get people back up, and you can often have one PC be downed and still have the rest of the party doing pretty well. If I'm running an enemy that wants to kill the entire party, and the party is trying to kill the enemy, having one at least person go unconscious is pretty common to make it actually feel challenging. But that doesn't necessarily mean that the party is actually in danger of losing the fight.