Despite being ranged, Ember Mage is the “tanky” unit of the Phoenix Elves, with the main role of covering flanks and blocking spots. Their attack isn’t good, but their major threat is being the potential source of Ember Beasts.
Be careful against factions that can do direct damage to units. Against direct damage, the Ember Mage is a very poor unit. Against those factions, it is better to use champions or even Royal Guardians instead of Ember Mages. Still, they are cheap enough that you can use them if entirely necessary.