This mob, while previously under-powered, now deals just as much as damage as a skeleton warrior, with double the chance of being the cause of your death. With the skeleton mage having a farther range at which it will begin to target the player, it can cause more damage to a player if left unattended during battle. Another point of note about these mobs is that, while being able to cause far more damage than a skeleton warrior, they have near half the health of one, making them easier to kill should you enter the fighting range to one.
When fighting in the spawn area of skeleton-type mobs, always keep in mind where they are, as failing to do so can result in one becoming a target to more than one skeleton mage. In these types of situations, unless of sufficient level to resist the particular mobs, it is best to rely on your potions or a fellow player to help in defeating your current enemies.
Although these are tricky to defeat, they are still manageable to be farmed for those level 50+.
They appear to be shooting projectiles using the book of nekros...