First off, welcome to the Shard! We’re glad to have you!
Ok, so Jhonnie’s Blessing sounds very good for the Watcher Knights, I’ve heard that’s what a lot of people use with them. If you wanted to, try combining it with other lifeblood charms, it will let you soak up ridiculous amounts of health while only focusing on attacking. I wouldn’t recommend hiveblood, you have to wait a while to get that mask back, and imho, it’s just not worth it to try and dodge for that long. I always take a couple of hits from the watcher knights, but you may be a lot more skilled at dodging than I am
Here’s a separate charm layout that I recommend for any boss fight, it lets you heal insane amounts of health while mobile, so you can pretty much cheese your way through most boss fights if you’re careful. It goes as follows: Deep Focus, Quick Focus, Shape of Unn, Grubsong, and Thorns of Agony. I don’t know if you have all of these charms (or enough notches), but the first three are vital for this charm layout to work; you can get rid of the last two if you really need them.
One last thing, if you didn’t already know this, in the room right outside the chamber with the watcher knights, you can mantis claw up to the ceiling, break it, and then find a rope you can cut to eliminate one of the knights before the battle even begins.
Hope this helps!