How can an adult child be protected against abusive parents? Domestic violence agencies won't help me

If the parents are physically abusive, contact the police immediately, inform any friends and other relatives that the person has and stop all contact with the parents by any means. The same goes for mental abuse although it could also be advisable to contact a GP, mental health practitioner, community organisation or a qualified therapist. If the adult is able to obtain employment and provide for a minimal standard of independent living in rented accommodation away from the parents then that would be ideal. If the adult is unable to relocate away from the parents and the abuse is very minimal, occasional and hard to predict then taking up some form of meditation, yoga or defensive martial arts could be beneficial as a way of calming the mind and strengthening the body against potential risks of future violence.