It's simple actually, allow "-1 gemstone spell requirement" effects to work with Ninja's catalyst stones.
Since they're pretty heavy and weaker(in someway) compared to other main classes.
I know the only available card for Ninja is Mistress, but hey, that'll make it even more "balanced" since it requires an MVP card.
...
Additional suggestion:
1.Casting X ele-Charms while under Pure Soul effect consumes 10 X ele-charm and calls 10 X ele-spheres
2.Release Ninja Spell will damage the target and all unit inside Cast Ninja Spell AOE and prematurely end it.
This is to improve Magic K/O, although it requires some preparation and alot of charms for the AOE to get big. Also renders yourself vulnerable due to Pure Soul for quicker preparation.