Which faction do you like the most?

The only good skill for mages is "Fire Fury," thanks to which archers deal additional fire damage.

Fire arrows are also available for rangers, and their archers are generally stronger. Goblins + Titans are worse than Archers + Druids.
By the way, I just checked the skill tree, and it says about this skill, "Ballista under hero's command...", but not a word about archers.