The thing is, defskull, you can't. The only way is to simulate the effect with triggers. The damage detection system won't be that helpful either.
Here is a Critical Strike simulation as an example.
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Trigger CS 1
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Events
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Unit - A unit Is attacked
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Conditions
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(Level of Critical Strike for (Attacking unit)) Greater than 0
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Actions
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Trigger - Add to Trigger CS 2 <gen> the event (Unit - (Triggering unit) Takes damage)
The above adds events to the below trigger, and the below trigger triggers the Critical Strike.
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Trigger CS 2
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Events
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Conditions
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(Level of Critical Strike for (Damage source)) Greater than 0
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Actions
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If (All Conditions are True) then do (Then Actions) else do (Else Actions)
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If - Conditions
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(Random integer number between 1 and 100) Less than 15
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(Damage taken) Greater than or equal to 1.00
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Then - Actions
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-------- Stops an endless loop of damaging --------
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Trigger - Turn off (This trigger)
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-------- Combined with the above trigger, this trigger multiplies your damage dealt --------
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-------- Since you already damaged this guy, you just have to decrease the life --------
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-------- Level increases multiplier --------
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For each (Integer A) from 1 to (Level of Critical Strike for (Damage source)), do (Actions)
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Loop - Actions
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Unit - Cause (Damage source) to damage (Triggering unit), dealing (Damage taken) damage of attack type Chaos and damage type Universal
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-------- Floating Text! YAYYY! --------
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Floating Text - Create floating text that reads (String(((Integer((Damage taken))) x ((Level of Critical Strike for (Damage source)) + 1)))) above (Triggering unit) with Z offset 64.00, using font size 10.00, color (100.00%, 0.00%, 0.00%), and 0.00% transparency
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Floating Text - Set the velocity of (Last created floating text) to 60.00 towards 90.00 degrees
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Floating Text - Change (Last created floating text): Disable permanence
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Floating Text - Change the lifespan of (Last created floating text) to 2.00 seconds
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Floating Text - Change the fading age of (Last created floating text) to 1.70 seconds
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-------- Stops an endless loop of damaging --------
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Trigger - Turn on (This trigger)
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Else - Actions