You sure that this RPG doesn't use GUI Damage Engine? -.-
It's really easy to use... i don't know why you don't want it.
You can actually improvise and calculate the damage - for example, you get the base damage of the hero (via object editor) you calculate how much damage the strength gives (in gameplay constants, strength gives damage, so when the hero gains level, it gains strength and with that, it gains damage) and finally you can calculate how much damage gives the primary stat of the hero (damage is gained not only by strength, but is gained by the primary stat of the hero aswell, intelligence for example). Then you store all this in a variable.
Example : You have a hero with 20 base damage, 13 strength (2 damage per strength point) and 7 intelligence (primary stat, 1 damage per int point)
So you set the variable equal to : 20 + (((Current strength of YourHero x 2)) + (Current intelligence of YourHero x 1)).
The damage of the spell is : 20 + 26 + 7 = 53. Then you can use the math function "In random number between X and X", and set that variable (53 in this case) to be lower/higher with a random number due to the object editor values "Damage - Sides per die" and "Damage - Roll dice".