Facing demon lords at full strength

[Luz]

Maybe the group finds out about her immunity and has to go on a quest to find some MacGuffin that can reduce or eliminate it?

Something like that. Maybe a magical device or creature(s) that empower her magic immunity like a battery. The PCs have to choose to destroy the device/creature(s) to reduce it or focus their attack on Lolth, much like a videogame boss.

 

[Eltab]

Rather than combat you could attempt to entrap the lord or lady with a binding contract. Be Iggwilv rather than a paladin.

Murphy's Law indicates that when you make the Arcana / Intimidation / whatever check to get them to sign the binding contract …
your d20 will come up 1.