Stay at range and kite the whole fight. This is not difficult unless you have bad players just standing in melee feeding him health.
The problem is that several good players who know what to do cannot compensate for even one bad player that refuses to adapt. That is not good for open world content. Basically, one bad apple shouldn’t be capable of spoiling the bunch.
Then teach them. Tell them what they can do, what to avoid and form a strategy. If they are newbies, guide them.
And if they refused to listen… well you can just tell them to kitten off and you guys can taxi to another map instance. Leave that unco-operative bugger behind