From what I understand, the demons in the Zelda series are not really something that exists in a way that allows them to be killed in the first place. They seem to be more entities, or corruptions of spirits that can manifest from time to time but do not really have a real physical form that can be permanently killed. This is why sealing seems to be the preferred method.
Take Lord of the Rings as an example. Maiar like Sauron were actually sufficiently powerful spirits that mainly existed in the spirit world but would in a way create a body for themselves so they could interact in the physical world. Now that body could be broken and killed like all living things but the spirit maiar was perfectly fine and could just as well make another body at a later date. To them it was more like tearing off their clothing that housed their spirits then outright killing them.
The still very undefined demons of Hyrule seem to work in a relatively similar way. Their physical shapes seem to be very malleable. With the mention that Demise takes on different forms depending on who is looking at him and Ghirahim who can reshape himself into objects, or Malladus who can possess the body of another person and corrupt it. They seem to be mostly an evil or corruptible spirit that can often take a physical shape that affect the physical world but can also be harmed by things like a sword when they are physical as well. This still does not stop their core self which is a type of spirit so that spirit that at first needs to be greatly weakened through a physical battle and then can be sealed away so they cannot return and make another physical self and cause trouble again.
Actually killing one for good does not happen very often and typically requires an item of incredible power to accomplish. If you think of spirits as a form of energy and energy cannot be created or destroyed then becomes difficult for any sword wielding hero to make any final blow towards them.
-Ancient Artifact of Benevolence (not necessarily the Title Item, but an item powerful enough to cause injury, or even death, such as Triforce, Light or Silver Arrows/Fused Shadows, etc.)
-Turning the villains' own attacks against them (i.e., tennis with Gannon, lightning against Demise, etc.)
But I don't think an exact weakness can be pinpointed. I mean, i'm sure that if Demise shot fire at you and you were able to reflect it back at him, it would hurt him. There's nothing wrong with your list, though, if your story calls for it.
All indications are that major demons have a sort of innate resistance to injury, possibly as a manifestation of their demonic auras. In order to do anything to them, that resistance must be overcome, whether through superior firepower or by causing backlash/feedback from their own attacks.