Doing automatical damage to attackers is a known principle of CRPG combat. You can use this principle yourself with certain builds/spells against the NPCs (like Armor of Agathys, Flaming Shield). Bosses are bosses because they are strong and offer a big challenge, so they have to have a lot of hp and strong abilities. They usually spawn minions if alone because it would be too easy in BG3 to deal with one char, even a very strong one, if you could concentrate on him alone from the start.
If the combat is too unpleasant, I would propose to change difficulty. I did it for example in the Isobel fight, from Tactician to Story. There is no need to torture yourself by high difficulty settings.