Actually, Kasha and Akara don't understand why Duanmu Huai is so anxious. In their view, fighting demons is a long-term task, and if necessary, spending years or even decades is not impossible.
But Duanmu Huai can't wait!
For him, one month at most, after which he must return to his original world! He can't waste ten or so years here, right?
But the problem is, there's really no way for now.
Duanmu Huai wants to directly go over and kill Andariel, then leave, but the problem is even if he defeats Andariel, there aren't enough people here to hold the Monastery. He might leave and then the enemy comes right back… getting into a war of attrition would be very frustrating.