The critical issues vis a vis such phenomena--in my experience are:
1. Does the person want free of demonic oppression/possession?
2. Is the person helping to free them confessed up, repented up, prayed up, maybe fasting and prayed up; anointed for the task; Grounded well in The Word and trained and experienced in spiritual warfare a la the Blood of Jesus; the Name of Jesus and the Word of God.
3. Are they prepared to be persistent.
4. How long the possession/oppression has been going on can be some clue to the degree of the challenge.
I haven't found a lot of other factors near as critical as the above.