If you really want to make this fight easy, separate your party and take Shadowheart in by herself to the make the blood sacrifice (I also closed the door behind her but I'm not sure if that matters). Once you make the sacrifice the door will be locked behind you. You can then try for a DC20 lock pick to open it and bring the rest of your party in with you. You will only have to fight a shadow copy of Shadowheart at this point with the strength of your entire party..