My advice is to go back to an earlier save and replay the Isobel fight (on Story mode preferably) till you win it because the consequences of a loss are for masochists only. I hate such game design but it is as it is.
I did that, and the key to success is...
Blocking all the doors, so that Marcus is on his own. That way, the winged horrors can't zero in on Isobel.
