In Byrgenwerth, I could summon the Younger Madaras Twin and Old Hunter Henryk to help defeat Rom, The Vacuous spider. However, both got stuck in the door to Master Willem's balcony thus they were unable to help. After Rom's spider bros exterminated me, I tried again with just Old Hunter Henryk. He got stuck as well, unable to pass the mist.
Note that I ended up defeating Rom, The Vacuous Spider easily enough without them. Others might encounter the same bug…
The exact same thing happened to me when facing Ebrietas Daughter Of The Cosmos and Mensis Scholar Damian.
How do you get those hunters to help you in the battle?
Do be aware that the boss's health does still scale even if the Old Hunter is not with you! This makes this a horrid bug.