This is a passive aggressive tactic I heard once, and it's pretty clever if you think about it.
1st. Prepare your case. Have on-hand several papers that have received A grades with you for evidence. Also, bring a couple of extra copies of your paper, with notes and highlighting that tell how you organized your thoughts, the paper structure, and works cited. Be able to talk about your paper whilst comparing it to your previous 'A' work.
2nd. Schedule an appointment with your instructor. Psychologically it is very difficult to be mean to someone when you are face to face.
3rd. Literally go through every aspect of the paper, from grammar
all the way down to transitional sentences. Frequently link back to papers where you were scored higher.
4th. Take your time. If you are being rushed by the instructor, then tell them you will reschedule. Don't allow the instructor to shuffle you out of the room. Be polite, respectful, but very firm. Your goal is to understand why THIS paper, which is very similar to MANY other A papers, received a lower grade.
5th. Keep reiterating this point. Your goal is to understand why THIS paper, which is very similar to MANY other A papers, received a lower grade.
6th. Use words like, "Help me to understand why, .....", and "I'm not sure why this (phrase, word, transition, etc.) is incorrect, would you explain to me why?"
7th. This is not, nor ever will it be an argument. You are simply trying to understand. Keep this tone throughout the entire interview.
You will (typically) find that the instructor will be much more reluctant to give you a poor grade as the class progresses.