Detecting just before a unit attacks...

I am tring to make a system where as a unit attacks, they get bonuses based on type of unit etc. So on unit is attacked event, i give the attaker a bonus damage ability, but unfortunatly the ability isnt active until after the unit already attacks. Any way to get around this?

* EDIT: Unit is attacked accually works as long as the unit already has the ability. I tried removing critical strike at the begining of the function, then adding it if the unit is attacked from behind. The way to do is let them have the ability all the time, and just change the level of it (level 1 being 0% chance hit, the rest 100%) then you can set the damage multiplier by making each level one damage multiplier higher than the next.

I use a system where I add bonus damage abilities on unit is attacked event, it works without a problem.