Are your spell leveling up with constant numbers(Level 1 - 50 damage, Level 2 - 100 damage, Level 3 - 150 damage etc) or random(Level 1 - 50 damage, Level 2 - 75 damage, level 3 175 damage)?
If it's constant, you can use a formula in the damage part of the trigger using the level of ability for triggering unit option, such as:
Deal ((50) x (Level of (Ability Name) for (Triggering unit)
This will set the numbers for the spell to deal 50 damage on level 1, but 300 on level 6. The numbers are of course easily changeable with some simple math in your head.