After a prolonged fight, Merra is already on her loose end. Beastman race may be blessed with strong physical attack and endurance. However, their mana capacity is worse than other races. Even their sixth grade still couldn't overcome this racial weakness.
Hence, the beastman will easily get tired whenever they fight using skill. Skill is unlike ordinary physical attack. Although some physical attacks look the same as warrior skills at first glance, they are still different regarding mana consumption.
Most skills need mana to execute. With the combination of Beastman's strong physical body, ordinary skills could become deadly skills in their hand. Unfortunately, this also makes the beastman tired faster as mana is still integrated deeply into their body.
Knowing Merra's exhausted condition, the Dwarves try to finish her off decisively. They didn't want this powerful sixth-grade beastman to create another disturbance later.