Cold Deer were much harder to deal with than the vampires had been.
At least vampires had obvious weaknesses; sunlight and clear water were both effective weapons against them.
But Cold Deer were different; Mu Ying had not discovered any exploitable weaknesses yet.
They had formidable strength and were equipped with abilities such as Extreme Despair, Bloody Frenzy, and Cold Gaze, which targeted the enemy's psyche, amplified their combat power, and remotely controlled the freezing of enemies, respectively. This made them formidable in both close combat and at range.
Moreover, both Extreme Despair and Cold Gaze were non-direct attack abilities, one targeting the psyche and the other a form of control, coincidently targeting the weak point in Mu Ying's defenses.