Well, that smug guy needs crushing. I hope it is sooner than later.
I do understand why Suimei seems rather weak here. He has been battling non-stop recently, so the toll of all those battles are getting to him now. First that demon guy, where he had to pretty much threw everything at him. Then he was hurt by that dark magic or something a few chapters ago. And now he has this battle too. Furthermore, his magic, while strong, isn’t that effective to demons and such here.
Perhaps this chapter is the author way of establishing that Suimei is not omnipotent. He is strong, sure, but he cannot fight forever. In fact, this actually proved his point in the beginning that it is foolish to fight against enemies who have superior number. No matter how strong you are, you will fall sooner or later against such number.
I think that now that he has more comrade, he might do better in the future. Hopefully, his teammate will pick up the slack.