Well, I'm not the best with invisibility triggers, but let me help you. Exists an invisibilty skill that makes the unit invisible and can't break it (I don't remember the name):
-Change windwalk spell to a generic [but instant] spell
-When activated, the trigger must be (sorry if I'm wrong with some words, english is not y native language):
Events
[Unit] starts to cast a spell
Conditions
[Ability being cast] is equal to [Name of skill]
Actions
Add [Permanent Invisibility] to [Unit]
(If you like to add a WW backstab probe this):
Events
[Unit] is attacked
Conditions
[Attacking unit] equal to [Unit]
[Attacking unit] has [Permanent Invisibility] equal to [True]
Actions
Remove [permanent invisibility] to [Unit]
Cause [unit] to deal [X] damage type [Magic] and [Universal]
Create a floating text that says [[X damage] + [!]] with [[R-100%],[G-0%] and [B-0%]] colors and [0%] transparency
Add [3] seconds expiration timer to [Last created floating text]
I hope can help you. You may wait for Ramza to help you