Accelerator is an idiot. If his power is based on vector calculations he could easily counter the 'pull-back' punch trick. Imagine that there is a layer around him that changes the vector to opposite when something touches it. he was hit only because he reflected acceleration only. when something is slowing down acceleration vector is opposite to speed's vector so he was hit (punch reached the layer and acceleration got reflected). but if he reflected speed, or even better force, there would be no way for such trick to work.

Overall this arc is pretty interesting, but when its getting closer to end its getting shittier and shittier - why Kihara didn't even try to do something to Index? oh random girl comes in and i'll just keep punching the Accelerator while she is talking on goddamn phone "how to destroy my plan". . .
Touma was retarded(like always), God's Right Seat... she is insane and interesting character but her reason for fighting is goddamn cliche and lame. Batshit crazy Accelerator was interesting to watch. Like with Croce di Pietro arc - it is really interesting at first but there is high possibility for ending of this arc to be utter shit too.