It took me some time to figure out who is the Boss - Bloodborne Celestial Emissary :) Boss is the one from those small alien octopuses (mushrooms?), the one who always stays behind. Damage him to 50% and now - run! - he will grow up. His stomps are not that dangerous, however with less than 25% of HP he will use arcane attacks. And stay away from him when he is going to blow up! Good luck and Have a nice game!
Got 2 runes from him: Communion and Moon Rune which increases amount of blood echos from all kills. Got to Trophies: Celestial Emissary trophy and Rune Master Trophy for getting Moon Rune, I guess. This boss fight is in the Upper Cathedral Ward - you need a key to open the door into here, search Youtube on how to get this key - a lot of guides have been published already.
