Originally Posted by geala
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.