As a Black Mage main, I have a certain rule I follow with non-boss mobs. "Let tank and melee handle the near-dead stuff. Bring misery to those full of life." Basically, as a caster, you should always be focusing your attention on the one with the most health. If it drops down below another, switch over. If they're dying at the same time, wait for both to get close to 20% and just simple AoE burn or stay in Umbral Ice.