I took the recommended approach of using god-mode but as mentioned I'm not totally happy about doing so even if it is a bug. My observation is that the bug seems to be not so much in the attack being overpowered but that there's so many of it, something like 6-10 in quick succession. Whatever attack it is it seems to be bugged in that it keeps on being executed instead of stopping after it's been cast. The same is true of both the Ba'al and Stone's battles but I think Zagan's minions are just OP as they don't have the same "machine-gun" effect.
