The fire spirit definitely needs to be weakened. It hits way too hard and the water spirit doesn't appear to deal any extra damage to it.
i agreed,quick strike also can increase the guage limit even if the sprite not taking any damage, but like i said it easier to leave it for last so ya can get alot of stat and hp increase,i also recommend ya spend the JP on passive skills
My recomedation of spirits are mostly water and iron as both can heal,whcih gives enough time and able to get the gauge limit faster,and when fighting them,pacify the lime spirit first as it can deal strong damage to multiple foes