[Autumn]

Having it have a cooldown would give stealing the buff a lot more worth, I'd say go for it, it'd be similar to how lunar lunatism plays out, where you have a window to strike before its activated again.

I'd say go for what feels right, but here's the ideas that I personally like:

Wraithguard only affecting magic damage

or

Wraithguard having a cooldown longer than its duration

and

Wraithguard being an evasive skill, and thus not stackable with Evasion.