Yeah, those black knights are pain in tje ass for unprepared player. In ds you meet them much earlier. Crystal sage is one of the easiest bosses in the game tho. And when you meet old dude with the hood in the church, you must know that it's an "entrance" into Dlc "Ashes of ariandel", it will be snowy area, so my advice is to finish the game first and then play the DLCs, otherwise it will be tough to kill even regular enemies in these dlcs
Edit: i bet you missed one boss in undead settlement, if you kill him, you will get an item that allows you to transform bosses' souls into weapons.