So I just need know how would you kill a demon lol I dono but I need to its for my book
Well let see, from the fantasy books I have read - They don' t like crosses or holy water. Some of the fantasy books have it that you can't truly kill a demon, only destroy its corporeal (physical) form - which banishes back to hell. Usually say after the the demon's corporeal form has received enough physical damage to it the demon will abandon it b/c it takes too much effort to keep it together.
so those are some common ideas I have seen in fantasy books. Although its your book, so you can bend and tweak those rules to fit your universe. I have seen lots of variations on those ideas
In many cultures there are thousands of different types of demons. Some of the common methods of killing them are silver and decapitation.
reflected sunlight from a cross! or a tapering three-edged spiked Bayonet.
The most commen ways are to decapitate it, burn it, or get an exorcism.
You can't kill a spiritual being.
vampire a steak in teh heart
Pimp slap it. :)