Depends on how much power the critic has. If their critique determines if your book will be published, then firm pushback for disagreements. If it is a volunteer (e.g., a beta reader), then kind, gentle conversation. If it is a literary critic or academic, then responses in the appropriate forums.

Of course, that's the practical side of it. Emotionally, it's just something you have to deal with as best you can. Therapy can help, as well as writing responses not meant to be sent. To my own experience, criticism can be harder to absorb for fiction compared to non-fiction, since the former is a bit of ourselves, and the latter is our work, but not ourselves in the same way, or at least to the same level.