It was true that the fireball keeps coming onto us when we were tried to attack the Elite Skeleton. Because of that, it makes us really hard to keep defeat the elite skeleton. The skeletal mage didn't even bother with Tane's arrow as his dead eye was locked on us.
We just kept trying as we thought that we might have a chance to attack it but there was not a slight opening at all. Tane also realizes that it was futile to keep shooting his arrows at the skeletal mage, he then decided to help us to defeat the elite skeleton first.
With that all of our attention was on the last elite skeleton, we thought that we have the chance to hit the elite skeleton now but the skeletal mage will burn down the Tane arrow when it tried to hit the elite skeleton. Vicar couldn't strike the elite skeletons as well.