I'll try to answer Grossuji, hoping I can end this here.
Can Kumagawa achieve infinite speed? Yes, he did it in the series. However, to do that he has to think, since All Fiction is not a passive skill. Since Dio's reaction speed is higher, this also mean that his thought speed is higher. Therefore, he can use Time Stop before Kumagawa can use All Fiction. If we were talking about canon Dio, this would be useless, since even if he kills Kumagaawa while time is stopped, Kumagawa would just come back to life with All Fiction.
However, HA Dio can simply erase All Fiction itself. If All Fiction is erased, it cannot be used to erase Dio's attacks, Kumagawa's deeath or anything like that. Therefore, HA Dio wins. If speed is equalized, Kumagawa may have a chance. If they're both in character and Dio decides to talk for two hours before attacking, Kumagawa may also have a chance. But if we're talking about a serious battle, whithout PIS/CIS and with speed unequalized, HA Dio could erase All Fiction before All Fiction can erase him or the time needed to move, so he would stomp.
Now you could say "But minus abilities cannot be erased, it was shown many times in the series", but in this case, we're talking about something that can erase it from the past, he could even erase Kumagawa before he obtained AF.
To confirm that this would work: All Fiction was born from Hundred Gautlets, a still that can revert cause and effect, that was transformed into a skill that can erase cause and effect. GER is the same kind of skill, but it still didn't work on HA Dio.
Trust me, Medaka Box is my favorite manga and Kumagawa is one of my favorite characters, I wouldn't want to downplay him, but it's almost undeniable that he would lose in this situation.