Its simple but very complicated. Since in your maps stats dont increase attack that method is out.
So you will need to make a complex system of data storage / recall for units and items, simplest method is using a game cash.
First of all you will need to store each hero's base damage under the heading of I2S('E000') where E000 is the raw data code of the hero.
Do a simlar thing with items and other attack modifiers.
When you cast the spells you can then reconstruct the damage the hero has by getting its base damage then adding each items additonal damage and you will get its damage which all you need to do is multiply.
You can also try other methods like the adding a critical strike method since that multiplyes damage.
Also the experimental method of geting it to attack a unit then multiplying the damage it deals and dealing more damage.
