Instead of saving the level of spell, why not you just save the actual value that needs to be saved ?
Like, you want to save the level of spell aura because you need to multiply it with 5, how about your store the calculated value ?
Also, does your map allows the unit possessing this aura to be more than one in the game's life time ?
Example:
Hero A comes near a unit, and gains Level 2 Aura.
Hero B comes near a unit and Hero A, both unit and Hero A gets Level 3 Aura (based on current Level of Hero B - the highest).
Is that scenario possible in your map ?