I'm giving you a choice

As the monster called Arthur continued to kill all in his way, the commanders of each battalion of soldiers from the different noble families had no choice, even if it will result in friendly fire, they needed to start to fire a barrage of spells on this monster. 

The stronger mages in the battalion started chanting, while they were doing so a lot of warriors protected them. They all risked their lives to protect the mages before they could cast their spells. 

Arthur in his berserk state did not dodge and simply took all the hits, but his bursting mana was helping him regenerate at an unbelievable pace. So most of his injuries even the fatal ones quickly healed. Not to mention it was very hard to actually harm Arthur, his body was sturdy and his burst of mana not only helps in regeneration but was creating a constant shield.