User blog comment:B214/Volume 23 discussion blog/@comment-187.15.164.191-20170506004927/@comment-79.26.121.225-20170510220313

Even Arthur trains every day, he practically thinks only to that, remember this. Rias attacking with Extinguish Star is fighting seriously, that is her final killing technique.

By the way they couldn't fight him all together, because their attacks would hinder each other. For example Rias power of destruction wouldn't allow to a close combat fighter to attack, it would be too dangerous, the same goes with Issei's attack, he could destroy a mountain so his comrades cannot stay even near to the explosion point. For example if 30 people attack 1 peole they will never be able to hit at the same time.