YAY! Loving how this is turning out.
I've been considering a Seramance for my next playthrough, but should I play it on easy (Adaar), smoshy (Cadash) or hard (Lavellan) mode?
The Lavellan/Sera romance is my favorite out of all the romances Bioware has given us, but at least I'm admitting to my bias before voting for you to join in. 
That fight was tough, though. I worked that out when I suddenly went "....why is Cass the only one standing?!" After that it was a battle to keep everyone else on their feet and still fight the thing. I don't think Cassandra ever fell once.
My first attempt at the battle resulted in a TPK, but my second had Whisper being the last one standing, with about 1/4 her health left, and then using her Knight-Enchanter Focus power to revive and heal everybody up to full. Rest of the fight was a piece of cake from there.