[Snake]

Too much of a hassle? Cap the maximum LV to 100, and the rest of the damage comes from 50% of your Fire ATK, so you don't go full SKI DEF VIT LUC and Retali-Narcus every round like an asshat, SO it works like:

On Take Damage: Store X damage (up to 100) + 50% of your Fire ATK, as Narcus LV.
On Hit: Deal unresistable Narcus LV >physical< fire damage.

The bad part on this weapon is that you don't even need STR to use it. *shrug*