What if defendant opposes this? Is there other method?
The respondent has an absolute right to maintain his or her doctor/patient privilege and to prevent you from obtaining the records. If the respondent has placed his or her medical conditions "at issue" then you can attempt to obtain them. Otherwise, you will not be able to get them and there is no other method for obtaining them.
It depends upon why the information is being sought. For example, treatment for drug and alcohol abuse can be obtained in order to determine a party's ability to care for a child. That's just one illustration. I'd need more information about what is being sought and why to give you a better answer.
I agree with my colleagues. You do not specify why you are wanting to obtain the medical records. If the health of the party is an issue in the case, then the records are relevant and you should be able to obtain them. Many of the Avvo attorneys on this site offer free consultations. Call an attorney in your area and let them guide you through this process. Hope this helps. Good luck to you...