It's possible the buff applied to all denizens of the dreadfort is being applied to the wither king, thus making him stronger than he should be, however, it is intended to be a big challenge, even for experienced players. If this is happening we will try and fix it. We have no intention of making bosses like this accessible for new players.
I don't feel like every mechanic and attack needs to be documented but some of the signalling in the fight, such as a player dying resulting in the wither king receiving a buff, could be improved to make it clearer what is going on.