What do you do about one demon?

[Harald387]

Originally Posted by Kromm

...and to the right number of PCs (you can halve 12 orcs for 6 PCs to 6 orcs for 3 PCs, but what do you do about one demon?).

This quote got me thinking about something our group brought up recently - how GURPS (and not just Dungeon Fantasy, but DF was what we were playing at the time) handles boss monsters. My experience is 'badly'; there's a line where the monster goes from 'sack of hit points getting mobbed by the group and beaten down in a slogfest' to 'unhittable killing machine that the entire group can't take', and I've never quite been able to find the middle ground. Has anyone done that sort of fight and had it go well, with the right mix of danger and triumph and some sort of back-and-forth in combat? What traits make this work?

(This is not a problem solely for GURPS - D&D 4's Solos are also notorious for falling into the 'sack of hit points slogfest' category - but I'm looking for ways to make it work in GURPS)