I would likely mix Cgarisma classes, Paladin or Warlock with Sorcerer or Bard.
But also I would rather try to buold them in Dtaw Steel or Pathfinder 2e first.
I never got that far, but from what you and others described it is a fight where mooks are niuisance that just heal the boss AND can be easily trivialized with I Win Button spells and focusing fire on the boss.
Then it really isn't a boss battle anymore, it's just a group encounter where the boss is not the main threat, just one of many. Your solution to disappointing boss battles is to remove the boss.
The thing is, that no matter what mooks do, the boss is either going to do more damage, having more negative impact, or the mooks will likely run away or disperse or disintegrate with boss' fall, so it is obiectively superior strategy to take out the boss first.
Yeah, I'm rather bitter about the label myself, especially after hearing how it and "cozy fantasy" have been used as drawers to put all women who write fantasy into, leading to harassment from the terminally online against women who include "uncomfortable" subjects like incest or suicide...in a...
From my experience pretty much nothing you listed will work.
Any debuff to the player character just works the same way as damage resistance or giving boss more hit points - makes the fight into a slog.
In my experience PCs will literally ignore mooks fireballing them to beeline it to the boss...
I hope they keep tension between victories and recoveries and how it affected the gameplay in new game as well, I felt it is PERFECT for dungeoncrawler
Raistlin Majere-clone time it is!
Seriously tho, a lot of games I played in where I got to roll had a houserule of "after you finish, sum up all roll results and divide by 6, if the result is below 8, reroll everything", so I don't think I could keep that 3 unless I had something big to...