Asisreo
Patron Badass
Could always give the extraordinarily tough monsters defenses against nearly all but 1 or 2 powerful spells needed in combination to incapacitate them.thinking more on this, I am worried the high level Combat fighter will still struggle to compete with high level combat magic almost no matter what we do.
Balors are one of the ones I look to often. They have teleportation and magic resistance. That's basically all a high-level monster needs outside of certain other saves and maybe immunity, resistances, etc.
People say the glaring weakness of the Balor is that it can be mazed, but a single Balor at high level is barely a hard encounter by level 17. Plus, it isn't horrible with intelligence. Having your one-of-a-kind 8th-level spell depleted to essentially buy time for something that may come back soon, it's not that bad.
Oh, and a player must first correctly guess the Balor doesn't have superhuman intelligence.
So more monsters like that would definitely help and I get to mention that the Balor is one of my favorite high-ish level monster.