Actually, Blizzard is the one who should be blamed ! (joking xD, but really...)
You see, I've been experimenting Shadow Strike spell and it works fine if the Casting Time is set >0 because if you set it to 0, it will deal an infinite amount of damage, causing any effected unit, dies instantly
The TRUE main reason is, Casting Time = Damage Interval
Well, I thought Casting Time for Shadow Strike is the "how long the spell is going to be casted before firing any effect from the caster" but it turned out differently
1 second is the minimum Casting Time you can set, other than that is futile
Conclusion: Set your Casting Time to a value that is greater than 0, and this problem is fixed.