Is the affliction of demonic possession or epilepsy a trial from Allah due to an abundance of sins and transgressions, and what are the most effective legitimate Islamic ways to get rid of it?

It cannot be definitively stated that afflictions are caused by sins, but Allah has legislated warding off بلاء (affliction) with supplication (du'a), ruqyah (exorcism/healing through Quran), and treatment. The afflicted person must be sincere, repent, and continue with ruqyah and treatment, asking Allah for healing and not being hasty for the supplication to be answered. There is no harm in seeking ruqyah from more than one reciter. Reading and listening to Surat Al-Baqarah (Chapter of the Cow) is beneficial in expelling devils.