I did beat them originally after 3 tries and a second play through
Heh, I beat them on the first playthrough on the first try with a L4 party, maybe luck? No special tactics, actually. I wandered into them, because they seemingly spotted me from offscreen
. Lae'zal hit them with menacing strike (two lucky hits), Gale with acid arrow, my Ranger simply with her +1 Longbow as a Hunter with Colossus Slayer, and Shadowheart with guiding bolt.
Of course, Gale died despite his mirror image and my Ranger had blind luck that three attacks missed her. It helped that they first attacked Lae'zal and Shadowheart, and then focused on Gale, and after he went down on my Ranger.
It certainly was a true WTF moment, which is probably what Larian intended.
