Kiriya acted very stupid. But he has to be given the benefit of the doubt. He probably did not know that Oga had demon energy that came through him because of Baby Beel. But the Zebel Blast attack was just awesome to say the least. I can’t imagine that being only a graze though. That also probably not the full strength of the attack either. But I would say that He wasted on Kiriya. He could have beat Kiriya without it. It was more to teach him a lesson so that way he would never return to St. Ishiyama.
I now of have a sneaking suspicion that Izuma has some type of demon power locked up in him. Or it could be that he has dealt with someone that had demon powers before. I just can’t imagine there being to many people having demon powers unless they are a demon. But at least Izuma was able to play off that fight as some type of show that they had planned. Now Oga and everyone has escaped being expelled.