In the final mission of the clone arc hero side ("Stop Protean's Invasion") Protean spams his Power Siphon non stop with no cooldown. It's instant cast, auto-hit, and results in him draining your endurance to zero while healing him to full. Makes it pretty much impossible to defeat him. Occasionally he will cast it normally (with the long animation, and the screen warning to get away from him). Then he goes back to spamming it like crazy after that.
Just ran this arc last week and didn't experience that bug with him at all, so I'm wondering if the latest patch introduced it. Anyway, hopefully it can be fixed soon. :)