Buff ability that adds X damage to attack

I am assuming that there is no built-in ability that will buff a unit in such a way that their attack damage is increase by 10 or 20. They all seem to just modify attacks by a percentage.

So it seems my only hope is to have a trigger that detects when a dummy ability is cast and add an ability to the unit based on Item Damage Bonus then have the trigger Wait for the Condition that the unit no longer has the buff and then remove the ability. Is that the best way?

Battle Roar, it can take integers in numbers or percent.

I think command aura can do it too, for the passive side of things.

yeah, but if it's a triggered spell Battle Roar is the way to go.

If you know JASS, you can find weaadar's (I hope the name is spelled right) Bonus Mod. This would be recommended if you are going to need to add many different amounts of damage/armor/hit points/mana to units, however if you just need to give 10 or 20, you'd probably want to go with darkwulfv's idea.

Thank-you that's awesome! I didn't know Battle Roar existed. Where is it used in normal Warcraft?

It isn't (not melee anyway, unless it appears on some random creep, never really bothered to check on that).

I think it's Rexxar's, not sure though. But yeah, Roar or Battle Roar is definatly a good choice, since it can increase many different stats (or decrease even)