Is it appropriate to ask the editor which changes he/she thinks is critical to the review?
I am not in your discipline (I'm in the health and social sciences). I have always learned that it's best to try (somehow) to address all of the reviewers' concerns. That being said, reviewers are not always clear (as in your case), do not make reasonable suggestions, or their suggestions conflict with other reviewers' suggestions. 

My suggestion is to revise how you describe the experiment and results. See how you can make your description of these things more clear. Then, when you write your response to reviewers' comments, you could say, "To address concerns by reviewer #2, we have revised and clarified the sections regarding our experiment and results. The time frame provided for the revision does not allow for additional experiments. However, if the editor feels that such work is necessary, we are open to further suggestions on improving the manuscript."

This way you do not look like you are disregarding the feedback from the reviewer and demonstrate your willingness to try and address his or her concerns. Good luck!