I actually prefer Sniper because It has 100% crit and other attack boosts. It does not really matter if the enemy have different elements because I use Mirel’s repaired bow (or Molten fury for nature boss). I do not need to heal much because the boss will already be dead by then. My playstyle only focus on 1 character which is Mirel. Others are supports like TP battery, Def down, Atk up, debuffs or healing.
Btw I think you focus too much on the magic. Weapons can also have elemental effects like Excalibur (Light) which is quite strong if there is a darkness boss.