I'm kind of glad that this chapter showed the MC using his head, and not going for the easy solution. Too many manga would either have him just go ahead and drop that giant water ball on the entire city. Either the day would be saved trivially--washing away the monsters, but somehow not the people--or he would have caused so much damage (played for comedy, in another sudden 180 reversal of tone) that he would be on the hook financially for all repairs. I'm pretty sure that I've seen both of these scenarios played out in other books before.