As a ninja I like stacking status effects, so on top of your poison you might want the spell that inflicts burn (from the mage) or the spell that inflicts ice (from the knight), but not both because they cancel. Alternatively you could use the life drain spell as your main attack to reduce the amount of time you need to spend on dedicated healing magic, but that reduces your damage output pretty significantly. Or I think the Windmill spell from the ninja itself is the highest straight damage per turn against enemies that don't have lots of magic resistance, and I'm not sure but it might give you three rolls to inflict poison from Snake Stance. If you've unlocked Ace skills, you want to assign one of those.
I like setting my Saint Heal trigger threshold higher than that, maybe 50 or even 60%; if you're riding closer to the edge consider moving the snake stance down lower and using one of your first two lines for a 'if enemy condition = charge, defend' to avoid getting one-shotted.
Gear-wise you want lots of MP, and MP regeneration, and magical attack. You can ignore physical attack and accuracy completely. Use whatever is left over for defense. Prioritize gear that gives a growth bonus above gear that gives a flat bonus, because a +2/lvl is going to be at least a +30 by the time you get to the fights that matter (outside of a small number of specific events that make you fight at level 1, which you'll have to respec for).
Oh, if you can, get the Mentality skill from... I think it came from one of the higher mage ranks, and then immediately after your healing line put in a 'if own condition is not mdef up, cast Mentality' to get a huge bonus to both your offense and magical defense. It won't help against physical threats (other than by killing them faster), but will be a big help against casters like the succubus.
