Not sure what level it should fall on, but reading a few other posts, I was inspired to think that a really flavorful Reaction for a Magus would be the following
FEAT ?
Retributive Parry Strike [reaction]
Trigger: You are struck by an enemy using a melee strike with a weapon or unarmed attack, and you have a Striking Spell stored.
You seize the opportunity to allow your Striking Spell to be triggered by the attacker. Treat this as if you had succeeded in a physical strike such as an AoO. If the spell requires a spell attack roll, roll your spell attack roll, as if you'd succeeded in the weapon strike. If the spell only requires a saving throw, have the target make the saving throw. The weapon itself however does no damage, only triggers the spell.