Duanmu Huai did not immediately head towards the location of the black shadow. From his previous encounters, he had noted that the shadow was extremely fast. Although it had been wounded before, Duanmu Huai was not sure he could catch up to it. So, he continued to choose a fishing strategy—after all, fishermen never return empty-handed.
Playing along couldn't hurt. Maybe the fish was stupid and would willingly take the bait.
So, Duanmu Huai pretended to be surrounded by the other side and retreated towards the other side of the street. As long as Mary was around, they could easily pinpoint the exact location of the monster. If the monster took the bait, it would be Duanmu Huai's time to strike.