Making them stronger/with more HP would only turn them even more into another Odin, which I'm already staying away from because it's simply impossible to even come close to.

Make them spawnable 24-men content that's exclusive to those who spawn it, like leves. To spawn it, you have to use 3 key items at roughly the same time (what they are and how you get them is part of the hunt). Whoever's in a party with the person who used the items is eligible for the fight and can't get another key item, but can still join someone else's party and fight the mark again.

Open world just doesn't work.