may be if posible a good idea could be making the spell be based on 2 variables that will add up:
using some random numbers as examples:
fixed damage increase: magery multiplied by 70.
% damage increase(based on weapon damage): magery multiplied by 0.25
100 magery=7000 base damage+25% increase weapon damage.
150 magery=10500 base damage+37.5% increase weapon damage
200 magery=14000 base damage+50% increase weapon damage
for wiz: couple with increase magic and some rares damage will be good enought
for alternative archers: they wont be having as much magery as wiz or increase magic but they will have better bows and there is where % damage enters and makes this spell worth.
with a bit of luck and balance

a change like this can break the only 1 build to go this classes are and bring more build options.