Hunter is pretty obnoxious as a wandering boss. You have plenty of gameplay features you are encouraged to use and he just... nopes out of all of them. He is completely non interactable and that doesn't make him at all 'fun' to deal with
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"
I'm guessing you never fought Puzzle bosses before.
The Hunter is one such boss also The Factory you cannot brute force those bosses. Being marked is a punishment actually since yes it take a turn however it make all the enemies attack and crit the marked target.
I Don't know what could be the cause of this.
But I once seen the Hunter with Haste.
I pretty much brute forced the factory, just get those pocket watches and a growth ray and then you can charm all three robots and even if they are very overcharged, they deal low damage while charmed and then you can just punch repeatedly the factory until it dies, reapplying charm and healing your heroes, the growth ray is so that a pocket watch carrier can actually deal correct damage to offset the factory healing.