Come to think of it, I recall skipping a room in the Defiled chalice, which had the dreaded butt-naked Watcher, after I heard it's not worth the trouble.
Here's a link for gem farming. The best gems can be found in the depth 5 root chalices.
https://www.reddit.com/r/bloodborne/comments/3469sj/toptier_gem_farming_spreadsheet/
I don't remember which of the first three on the spreadsheet I used for farming, but the Merciless Watchers in the Ihyll chalice are good source for +27.2% physical attack gems. I think the best option is to have Kin damage down curse, since there aren't many enemies in the game that are affected by it, or stamina penalty.
Regarding Kos, getting so close to defeat a boss can be infuriating, just like barely defeating one feels exhilarating. It's the only boss fight, where I've used the Moonlight Sword, and its R2 swipe works well in the second phase. The NG+ encounter was less troublesome for me:
https://youtu.be/iI4uOVejx8g
EDIT: The Winter Lanterns just before Kos are also a good source for +22.3% physical attack gems.