This is a bare minimum trigger, should work, didn't test, GUI isn't really what I do xD
Mana Burn
Events
Unit - A unit Starts the effect of an ability
Conditions
(Ability being cast) Equal to Your_Ability
Actions
Set Target = Target unit of ability being cast
Unit - Set mana of Target to (Mana of Target) x 0.75)
Custom script: set udg_Target = null
// 2 Lines // 1 GetUnit()
Set Target = Target unit of ability being cast
Unit - Set mana of Target to (Mana of Target) x 0.75)
// 1 Line // 2 GetUnit()
Unit - Set mana of (Target unit of ability being cast) to (Mana of (Target unit of ability being cast)) x 0.75)
JASS:// 2 Lines // 1 GetUnit() Set Target = Target unit of ability being cast Unit - Set mana of Target to (Mana of Target) x 0.75)
JASS:// 1 Line // 2 GetUnit() Unit - Set mana of (Target unit of ability being cast) to (Mana of (Target unit of ability being cast)) x 0.75)
In this case where the (Target Unit of Ability Being Cast) is just being used twice, it doesn't matter. But if you have more functions involving the same unit, using the variable is waaaaaayyy better.