Set TempReal = (Real((Strength of (Triggering unit) (Include bonuses))))
Unit - Cause (Triggering unit) to damage (Target unit of ability being cast), dealing TempReal damage of attack type Spells and damage type Normal
TempReal implies to a variable.
Real is a real number like 0.01 or 21.00, basically everything you use to calculate etc
To set it up, open the variable editor (ctrl + x) and add a new one, name it TempReal.
Now go back to the trigger editor and for actions, click on Set Variable.
Choose your variable and confirm.
Now you have to set the value it holds, which is: (Real((Strength of (Triggering unit) (Include bonuses))))
The strength (e.g. 18) is a integer.
The action Real(...) converts the parameter (the stuff inside the brackets) from x to a real - in your case choose Mathematics - Integer to Real.
Now, the (Triggering Unit) is the unit which refers to the given event, like
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Events
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Unit - A unit dies
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Conditions
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Actions
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Unit - Remove (Triggering Unit) from the game
(Triggering Unit) works in the most cases like 'starts the effect of an ability' or 'dies' AND it works after waits.
Now we have to say that things like (Target unit of ability being cast) does NOT work after a wait, so you have to set a variable to it (see above).
Now the second action Causes the (Triggering Unit) to damage the unit on which the ability is casted be the amount you choose.
Here, it's TempReal, so basically the Strength of the unit which casts the ability.
Further details?