Just hit Terror Baal, and was dominating all the way up to that point with my necro summoner. For any other build, that telegraphed black-hole looking attack would be fair, but definitely not with a summoner.
Whole minion army is instagibbed. I could ween him down, jump back in portal when army dies, rinse and repeat a numerous amount of times, but that's just dumb. If the attack is magic damage, I would suggest that next patch the minions get a sizable resistance to it, since the AI has no qualms about facetanking a telegraphed instagib attack.
My minion stats at lvl 111 (obviously endgame needs more)-

I should at least say that i'm still really enjoying myself this patch.