Enemies with no weaknesses are not cool Larian (except for Madara) ... they are only frustrating.
I am sympathetic to your argument in general, but I found the "Grym, Eternal Protector of the Forge" fight to be enhanced by its complete and utter lack of weaknesses. I don't like how DnD can almost default into a core gameplay loop of just lowering HP with whatever moves you have. Enemies without weaknesses can be fun by forcing you to get creative outside of how you do other fights, as long as they incorporate some other mechanic that gives you a fighting chance.
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He's extremely vulnerable to any Bludgeoning damage as long as he's been heated by lava in the past two turns.
And very easy to kite around with with the way he's always aggroing the last hitter.
These are textbook cases of vulnerabilities to exploit. Also, if you manage to drag him on the center of the platform he's pretty much done.
