A hero's attack damage is based on his main attribute and then a random dice roll decided by a number of dices and the sides of each die.
You can find the dice roll in the object editor under your hero. It should be about 1/3 down, right above the hero's attack type, range etc. If for example, you've input 2 dices and 6 sides per die, it would equal to 2-12 damage. Lets say the hero has 20 points of his main attribute, his damage should be 22-34.
Assuming you have a basic understanding of triggers, I'll go ahead and free hand it for you;
Events
A unit starts the effect of an ability
Conditions
Casting unit equal to (Your hero)
Actions
Cause casting unit to damage target unit of ability being cast, dealing (your base ability damage) + real(intelligence of Your hero (include bonuses)) + random real number between 2 and 12) damage of attack type spells and damage type fire
lol... what when hero will get 6x claws of attack? It won't be even similar to hero's damage