I have not been there yet on that quest, but i have found easier ways to get around killing larger groups of hard mobs by using your surroundings. I did not consider the environment in this game to provide as much support as it can, but then, being desparate, I tried everything... Like in the back alleys in Denerim... I had to hold my whole group up on top of the platforms with the 2 tanks blocking the entrance, and the caster behind them.. Then I (elf ranger) would pull groups 5-6 at a time and then hide under the ramp that lead to where my party was.. They would stop hitting me and run up the ramp to my waiting party. Then, i'd just peak out and shoot them in the back.
Hide casters behind objects that obstruct their line of sight from other ranged enemies so they can provide support to the melee in the group while focusing on healing your tanks. Then once the melee is done, i let my party walk freely to destroy the remaining ranged because they are a joke..
Sorry my help might not be very specific to your situation, but might be helpfull if you can find a ramp or platform, or decent cover to hide your people behind..