I employed a simliar strat when fighting the Hag Witch, only this time I went with the Highwayman instead of the Crusader.

The boss only landed 3 attacks the entire fight, and the party walked away with no addtional stress, and needless to say the run was more than profiable with the Antiquarian. The Crusader was able to make quick work of the boss with the Necromancer's Collar. However, Once we got into the boss, after a few rounds of ManAtArms spamming Bolster and the Antiquarian Spamming Invigorating Mist, it was a breeze: since the anchor couldn't land the boss has no health regen.

By the time we got to the boss the Vestle was at 80 stress, and the other 3 memebers was around 40%. I had a rather diffcult time before the boss as the Crusader was agonizingly slow as a DPS, and there weren't sufficient damage to the back rows. I went Antiquarian-Crusader-Vestel-ManAtArms on the Sunken Crew (Champion Dungeon). The middle two are based on the dungeon/mission. The base formation is Antiquarian(Bag of Marbles, Feathered Charm)-healer/off-healer-DPS-ManAtArms(Quickdraw Amulet. Ofcourse, I am playing on Radiant (filthy casual, yeah) but anyway here's how it works: Recently however I found a viable way to bring the Antiquarian into Champion dungeons and boss fights. People generally use her for farming only and advise against bringing her to champion difficulty dungeons and boss fights, and most people who recommend her says she is only useful in specific senarios (i.e. Personally, I think the Antiqurian deserves a bit more love and a lot more credit than people give her.

Katherine antiquarian darkest dungeon