Lowering life via triggers?

I can't find how to do this.

It goes something like this right?

Set life of (Event Response - Target of Casted Spell) to life of (Event Response - Target of Casted Spell) -50

But I can't find any action that allows me to add the "-50" at the end.

Can't you use arithmatic like . . .

Set life of (Event Response - Target of Casted Spell) to life (Arithmatic - (UnitHealth) - 50))

Where UnitHealth is a variable counting the target's current life set just before the health action.

I believe it goes like this:

Unit - Set life of (Target unit of ability being cast) to ((Life of (Target unit of ability being cast)) - 50.00)

Yah, dragon, your problem is that your not using Arithmetic. Arithmetic allows for 2 #'s that can be added, subtracted, multiplyed, or divided by each other. And if you need multiple #s to use, just use Arthimetic (5) + Arithmetic (5 + 5)