Having a CR 20/24 PC in a game with lv cap of 12 would't work imo.
True, but considering powers levels of bg3, we are way past CR 16. Raphael 666 hp being higher than a CR 30 Torrasque (or close to it) with higher AC.
As it is, The Slayer is weaker than a Young Red Dragon. And elder brain only has got CR 14, while in BG3 it's obviously higher due to insane power creep.
To put into perspective, a party of 4 level 12 adventurers can fight a CR 17 monster as a boss and that's in tabletop
