Now Peter has a floppy hand, nice. He'll worry about trying to make a fist later, right now he's kinda busy getting punched in the stomach. There aren't any new flames coming at him so he's not too concerned with dodging, and he's kind of a superhero with abs of steel, so there's not a lot going on in the reaction department. What he does do is lunge forward suddenly, mostly just with the intention of body slamming Gokudera and knocking on his butt.

"So, just in the interest of keeping this fair, I once got hit by a train and walked away from it? so just feel free to hit me with whatever." It'll be fine, literally annihilate him.