Returning to Yahar'gul and figuring out the quickest shortcut back to the boss, I downed The One Reborn after a couple of attempts tonight. Obviously you've got to take care of the bell-ringers before you face the boss itself - an Empty Phantasm Shell-buffed Kirkhammer and a stab to the gut took care of each with no problem. The boss itself, which so far is the grossest I've encountered (I have not done Ludwig yet,) took a couple of attempts to figure out. Three or four smashes to the... foot thing? with the Logarius Wheel was enough to stun him, and then I slid out (actually, you sort of have to figure out how to do this, as it mostly feels as if you're stuck) and bashed him on the head a few times.
Getting away from any area-attacks including his rain of corpses, I'd duck under the cloisters and skirt around him, attacking that foot-thing from the side to repeat the process. Two or three of these cycles was enough to take him down.
So then I went into the Lecture Hall floor 2, met up with Patches the Spider before proceeding on to the Nightmare of Mensis.
The Nightmare is appropriately nightmarish. The beast-like monsters, similar to the ones in the Nightmare Frontier, spawn some hard-to-hit and hard-hitting little worms (similar to some found in Cainhurst... reminds me, I have some treasure to collect there.) Then there's the frenzy you get just from looking at the bridge (though thankfully, perhaps because I only have 15 Insight at the moment, it wasn't enough to cause me any real trouble.)
Anyway, I soon found out that my best bet was to run the hell through that area, dodging beasts and boulder-throwing giants. I would have run past a room with some massive spiders, but the next contains a rather tough Church Hunter who demands a lack of distraction.
Thankfully, I was able to unlock a shortcut that takes you right to a lantern, from which you have a clear shot to Micolash, Host of the Nightmare.
What a weird fight. What a weird and infuriating fight.
I haven't yet gotten him down, though I did have one of those painfully-close attempts where I think a single additional strike would have defeated him. You can kind of describe the fight as having four phases. Phase one, you have a rather small area to chase him around, though there might be a set path to force him into phase two. You need to chase him into a dead-end room to get him to stand and fight you.
Inside, there are two puppets that I think either don't respawn or at least take a long time to do so. During this phase he literally just does Augur of Ebrietas, so the best thing I've found is to try to bait it out and sidestep, then punish him with a few smacks before backing away.
Once he hits 50%, he disappears, and a larger part of his boss arena opens up. This one is far more labyrinthine, and I came across a ton of different treasures just trying to find him. Eventually, you can get him to hop down from a balcony, and if you circle around to the opposite side of the room from which you enter, you can force him into a room that will lock.
Going upstairs after this, you can find another room with a smaller balcony-overlook, and hop down to fight him again. This time, however, he has melee attacks and, most annoyingly, A Call Beyond, which hits very hard, is nearly impossible to dodge, and stuns you long enough to keep you from stopping his next cast.
I found that hitting him before he could cast it tended to be a good way to survive, but between Augur of Ebrietas forcing you to keep your distance and his melee attacks, it's not terribly easy to do. I'm hoping there's an angle where you can always get a falling attack, but for now I'm going to be equipping the full Choir set, as it appears to have the highest Arcane defense among my gear.
I'm getting pretty close to the end of the main game. I think I'm going to stop after Micolash and save Mergo's Wet Nurse until I've taken down Ebrietas and the Old Hunters content. I also still need to (well, should) do more Chalice dungeons, though I'm not entirely sure where to get the appropriate Ritual Blood to do so. I also need to collect items including Umbilical Cord Thirds from fake Iosefka and Arianna, though I'll need to check and see if those are available yet.
But for now, I need to farm Blood Vials. Man, I had like 70 of them.