OK, here an idea:
Create an Real Variable for your hero (here called it "WarClubHits"), as long as there's a limit for every player on how much heros able to cast that ability he can build. Without limit, it's very hard to create enough variables
WarClubCreateCastRefer
Events
Unit - A unit Finishes casting an ability
Conditions
(Ability being cast) Equal to [Your Ability]
Actions
Unit - Replace (Killing unit) with a [Unit with Damage Bonus] using The old unit's relative life and mana
WarClubHitCounter
Events
Unit - A unit Is attacked
Conditions
WarClubHits Less than 15.00
(Unit-type of (Attacking unit)) Equal to [Unit with Damage Bonus]
Actions
Set WarClubHits = (WarClubHits + 1.00)
Wait 0.01 seconds <- don't think it's necessary, but simply added a wait action
If (All Conditions are True) then do (Then Actions) else do (Else Actions)
If - Conditions
WarClubHits Greater than or equal to 15.00
Then - Actions
Set WarClubHits = 0.00
Unit - Replace (Attacking unit) with a [Unit without Damage Bonus] using The old unit's relative life and mana
Else - Actions
Do nothing