XombeR, A unit is attack is NEVER a solution for a damage-detection. This simple trigger here allows you to use teh event in any trigger you want.
Damage detect
Events
Map initialization
Unit - A unit enters (Playable map area)
Conditions
Actions
If (All Conditions are True) then do (Then Actions) else do (Else Actions)
If - Conditions
(Execution count of (This trigger)) Greater than 1
Then - Actions
Trigger - Add to [Your trigger] the event (Unit - (Triggering unit) Takes damage)
Else - Actions
Set TempGroup = (Units in (Playable map area))
Unit Group - Pick every unit in TempGroup and do (Trigger - Add to [Your trigger] the event (Unit - (Picked unit) Takes damage))
Custom script: call DestroyGroup( udg_TempGroup )