It's not that the Blood Sacrafice deals alot of damage, it's that if the attack is successful, it will drain everyones prayer by half. Not something that's really nice to tank out.
Just something to throw out there, but I'm reasonably certain that your maxhit with any weapon caps at 500. I've used quite a few BGS specials on her, and my max with it was a flat 500, 707xp Special.