I personally don't really consider that a flame. A flame isn't simply someone that disagrees with us or dislikes something we've written, contrary to what many people believe these days. A genuine flame is where a person makes rude and nasty comments with the sole aim of causing arguments and upset, and with absolutely no trace of constructive criticisms in evidence. While that critique was delivered more harshly than is strictly necessary, the person is giving reasons why they dislike certain parts of the story, and, without being at all familiar with the fan fiction, they do appear to be valid criticisms, at least to some degree. I think what the person was trying to say, in a rather rough way, is that you might need to offer better explanations and descriptions in certain parts of the story. At the very least, it might be worth rereading the story slowly and carefully to see if the person might perhaps have a point here and there.
It's also a good point for reviewers. If you deliver a review harshly, no matter how warranted you might think it is, the chances are that the author will simply dismiss your comments out of hand. If you're going to leave constructive criticisms then it really pays to do it calmly and rationally, fully explaining exactly why you disagree or dislike what was written. Otherwise, you're almost certainly waisting your time leaving a review.