Biggest issue with the fight is it's very dependent on how many adds come into the room right away vs get caught up fighting npcs. If you don't know it's coming, well chances are the door will be wide open and they'll rush her.
You want them all balling up at the door for her. Because then your Gale with evocvation's spell sculpting casts a Fireball on them, it doesn't hurt allies, and one shots the entire set of fiends while leaving Marcus as the sole remaining opponent.
