What is the best treatment for a severe pressure sore?

Wound care specialis. See a wound care specialist or center because pressure sores may require a multi-disciplinary team to enhance nutrition, control contributing systemic or local disease, institute physical therapy, recommend appropriate medical regimens or treatment protocols including possible surgery.
RX pressure ulcer. A location and stage would help answer this question a little better. But direct observation of the wound is absolutely necessary to answer this question. Minimize the pressure, debride the devitalized tissue from the wound, use clean products in or on the wound, and careful followup by wound care professionals either in a wound center or by home health nursing is vital.