Yeah not interrupting that is pretty much the only way to wipe in FE anymore. It has such a long casting time it is almost impossible to miss, but I have seen many tanks just ignore it in pugs. Usually do this with a guild friend; we have taken to giving each other heart attacks in this fight. If playing my shadow I will wait to see if the Tank gets it until the last second. My friend playing a trooper healer will be freaking. If I am playing on my sawbones, he will do the same thing to me. Surprise how often we do this, because when my same friend is playing his shadow tank you hardly ever even see the cast bar, he hit interrupt when the animation first starts.
This made me laugh -- that is pretty awesome.
Incidentally, there is a buff icon
that appears before he casts it. I've accidentally hit my interrupt right when I saw the icon, but before the cast bar started, and it actually interrupted it.