I'm mainly playing as a warrior but still here are a few tips I can think of:
- the Wraiths (spirit bosses) are pretty hard so increasing your overall level is definitely a good idea
- grind the opposite element as much as possible (I mean if you want to fight the Fire Wraith, then level up your water magic)
- equip the best wand/staff you can and attack from distance
- bring two allies with you to help, maybe melee fighters (like Odin) so that you can use them as tanks, just revive them when they faint
- bring as much as potions as you can, especially life cures and the ones for SP recovery. It's probably best to use earth magic to heal yourself, that way you also level up your overall magic skill.
Optional: play the Alchemist life too as it complements the Magican life (you'll be able to brew your own potions, including "super" ones which are more efficient the regular ones) and/or the Carpenter life to make more powerful wands