So I'm almost done with my first playthrough of Bloodborne, and will start NG+ soon enough. On my first playthrough, I've been hitting bosses with pretty much pure physical damage because I had no idea what were their weaknesses. I could obviously just look online at each individual boss' resists online, but I'd like to avoid that. I'm more interested in general rules for bosses weaknesses based on lore.
For example, Cleric Beast, Blood-Starved Beast and Vicar Amelia all look like they are the same "breed" of monster, so I would assume they are weak to the same element. The One Reborn and Amygdala seem to be pure nightmarish products, so I'd assume they are the same breed and have the same weakness.
Is there a general rule for boss weaknesses in Bloodborne that would be based on the breed or type of boss? Instead of just adding random elements to my weapon, it would be good to add the appropriate one based on the boss' type, looks or backstory.