Interesting question.
I would *LOVE* to have some sort of supernatural encounter. Almost had one once, but I quickly found a rational explaination -- and felt pretty silly at the end!
I would note, IMO, your logic is flawed. If you experienced a "demon attack", that would prove the existence of demons -- and nothing more (actually, it might just be the result of Sleep Paralysis and be a figment of one's confused brain). True, if there were "spirits" then there could be more spirits of varying degrees (i.e. Satan, God) but you would still not have proof of Satan & God.
For example, what if these Demons encounted are the ONLY spirits (i.e. no angels, no Satan, no God)? What if they invented the concept of God and Satan (in the same way that I believe that humans invented God and Satan)???
Having proof of the existence of Demons does not prove the existance of any other spirit creatures... IMO
~Quotes, of the "Careful with logic" class