Part 84: Feinne's Notes: It's Benny, NatchUnkillable: You Can Call Me Al, Episode 3: B-B-B-Benny and the Jets
And now we've met Benetnasch/Alkaid, the seventh of the Septentriones. He's kind of a jerk here, since this is intended to be an unwinnable battle. The demons before you engage him aren't terribly threatening, just clear them out and get your skill cracks. I'd personally suggest engaging him with Evil Flow, because it lets you start the final phase of the map from as far the hell away from him as possible. You really don't want to be in a position of taking hits from Benetnasch that you don't have to, because you don't have any demons to help. Making sure the character who pulls him is immune to physicals is also pretty useful, as noted.
This map actually does a good job making you concerned about Benny, but we'll have to see how that pans out when we finally get to fight him for real.