My issue is less that its 'possible' to kill him (I exploited his behavior and slurped him down with red), and is more that he isn't that interactable as a boss.
Though there is one point that while probably not intended is a good baseline to improve the hunter If you use taunt he immediately marks his intended target, this is GOOD your taunt has the intended effect it saves your ally and forces him to use an action to grant a debuff, a moment you can use to overheal your ally and prevent the big damage the next turn, (or find a way to clear the mark and then taunt to get another round, but this probably wasnt intended) he spends an action [bonus for exploiting his behavior] and threatens you with worse [lets you decide how/if you are going to defend or eat the damage]
Everything else he just, 'nopes' out of wasting your turns on trying anything besides "BIG DAMAGE NUMBER"