Cultural relativistic answer:
how does the devil able to enter/inhabit the body of a human being without even using tools such as spirit boards?
Demons are quite capable of entering hosts without any sort of medium. There doesn't need to be a reason.
Can the person fight it off even without performing exorcism?
No.
.....
I've always wondered at the connection between psychosis and possession, and how deeply religious persons sort of become self-possessed. Or not deeply religious really. I mean in psychiatry there's a whole slew of disorders and labels that virtually mimic demonic possession. I think it does stem from a sort of cultural background where a person becomes fixated on the idea that they are possessed, ie mania. Its a neurological disconnect brought on by fever or acute psychosis. Even a traumatic brain injury can mimic symptoms of possession, or epilepsy.
Let's take a look at the symptoms:
erased memories or personalities - coping mechanism of trauma, avoidance behavior, PTSD, dissociation, common in victims of severe abuse
convulsions, “fits” and fainting as if one were dying - ? acute anxiety, insomnia, high fever, delerium brought on by illness
access to hidden knowledge (gnosis) and foreign languages (xenoglossia), - mania, prepsychotic behavior involving intense study of another language or related materials, even the study of demons, etc.
drastic changes in vocal intonation and facial structure, - psychosis with mania related to possession, self serving behavior
the sudden appearance of injuries (scratches, bite marks) or lesions - self harm, acute illness
superhuman strength - psychosis, fight or flight response, neurological injury, "fear" - possible panic disorder - anxiety
I think what's more at issue is the Catholic Church's borderline stone age perspective on these issues. They are operating very much on an old, historical method that is dangerously ignorant:
http://en.wikipedia.org/wiki/Jeremy_Davies_(exorcist)
His book, Exorcism: Understanding Exorcism in Scripture and Practice was published in 2008 by the Catholic Truth Society. In the book he warns against the New Age, the occult, promiscuity, homosexuality and pornography saying that involvement in these things can lead to dire spiritual consequences.[2]
I mean really? Being gay is an invitation to demonic possession? Its like they are buying into their own bullshit.