GW2 scaling (as far as I remember from playing it briefly) was to increase the number or difficulty type (veteran, champion, etc.) of mobs. Here, we’re only talking about one mob.
I’m suggesting increasing the health pool proportional to the number of players nearby. I’m sure abilities such as personalized AoEs (I’m thinking of cave-in mechanics, underground spike mechanics, projectile volley abilities, etc.) could be coded to target any nearby players such that they auto-scale. As long as the health pool scales proportionally, the fight couldn’t be won via zerging.