Dean very nearly responded then, very nearly said something back to her...but then she started talking to herself. Talking as though she were having a deep conversation with someone, but he could only hear one half of it. He stared at her for a moment and finally spoke.
"I am a Gryffindor and you killed someone important to me. If I'm right, you killed more than one person important to me. I'm not playing the hero. I'm just not playing your games. You-"
He would have said more but she hit him with another spell. This time, since his mouth was open and he was in the middle of speaking, in earned another gasp of pain. It hurt, and he tried to double over, but pain was racking him and moving made it worse, not better.
He had to figure out how to keep her from killing him, but keep her off-guard enough to make some kind of escape. He couldn't take her out without a wand, but he had to get away.