I've defeated this on Legendary with the following group: Kyra, Harsk, Valeros, Merisel, Lem, Ezren. This is a sub-optimal group for this scenario.
There are lots of bugs and some things don't work. There are a lot of things you can do though:
Blessings of Cayden Cailean (lots; Unity is best if you can get them)
Blessing of Sarenrae (Kyra only, good for closing locations)
Greater Aid spells (Kyra, Harsk)
Ring of Serene Contortions (x4)
Greater Sage's Journal (x3)
Belt of Physical Might (x1)
Lots of search cards
Lem (is a boss)
Valeros (Guardian w/Shield Another, very helpful cuz shaves off a lot of damage from movement)
I kept everyone grouped together. If possible I tried to have only Valeros, Harsk or Merisel do henchmen fights. All of them need help regardless but each has their advantages.
It did take a lot of tries (some due to bugs) and a significant amount of deck retooling. I've since rebuilt these decks so I might be missing a few things. Its unlikely I'll do it again because it is obnoxious. Every other scenario is significantly easier on Legendary.