I somehow managed to beat this fight by sheer dumb luck (or possibly because of a bug) the first time I found it. After the first couple of rounds only Astarion was left standing (Gale, Lae’zel and my cleric main character all died). I had him and Gale on the bridge. Gale pushed one of the Gith to their death before he died, then Astarion pushed the other. Then I put Astarion in stealth and slowly chipped away at the remaining ones below. All they did during their turns is yell threats; no attempts to look for the hidden rogue above.
I somehow doubt I could duplicate the surprising win if I tried, but maybe the stealth height advantage thing is worth a shot.
Yeah, I took the Hag out in an unrepeatable way. Still not sure how it happened either. Ah well, will plod on.