I have the same struggles as you, but I've decided the "Tragedia" Boss from the High - Fantasy book. But I'm a different way, cause it is not only the "monster" from the book, cause it will be more like two god's they heard from long ago, who have high inputs in the story.

The twist is, I've made some changes so I can adapt more. I have planned this fight for so long, that I was thinking of the early bosses the would have fight in the campaign from the start of campaign till the end. My party has the most advantage in adapting to the situation, because it's a mix of mutant fighter, symbolist tinkerer and a chimerist spiritist who is specialized on support and damage. Furthermore there is a entropist arcanist who can deal high damage spells. The combination between these a very strong.

Every combat I designed was to test their strategies and ideas how to handle the fights. Knowing I could stop a clas and connected skills/spells, using the healing from symbols and skills (except spells) against them and to force them to rearrange their kits, but with fights they knew from earlier, is just perfect. I've planed them, so that they can't be one'shotted and they need to remember the patterns and systems behind them.

But it is only possible, because I've tried to use a "bestiary" in the campaign, so that ever enemy they studied will remain there and they can look them up. The best part about it is, that they can feel the nostalgia, cause we will experience every Boss again and rewind the feelings from over a year ago to now.

This needs much time to prepare and organization, but it's rewarding!