Jellylala is exactly right, the key to this boss fight is to shoot the orb on her chest. This is easiest if you are playing co-op with another person, and not just the AI. Have the player who is controlling Sheva walk up behind Jill and restrain her.
Once Jill is restrained, Chris should run around to the front, but instead of hitting the button to try and tear off the orb, you should use your handgun to shoot it. On normal you will have to repeat this about 3 times, on veteran about 4. You will know that you have shot the gem enough to eventually succeed in removing it if she occasionally gets shocked by the orb.
Even though you have shot the orb several times at this point, Chris will have to attempt to pull it off about 3 to 4 times anyway. The reason I say you should have Chris do the shooting/tearing is if you succeed in removing the orb with Sheva restraining Jill and Chris pulling instead of throwing Jill to the ground or having Chris do the restraining, you will unlock the "Masters of Removing" achievement and get 30 gamer points (on Xbox) I imagine there is a trophy of the same name for PS3.
You have to just keep hitting the A button. It took me around 8 or 9 tries and it finally came off.
It's frustrating, but keep at it. Just keeping mashing, eventually you will loosen it up enough to pull it out. It took at LEAST five time (sometimes it seems like 10) to get the stupid thing out.
Try grabbing Jill yourself, sometimes the computer partner will shoot the orb, that seems to weaken it some. Also gives you the added bonus of shocking Jill randomly, which makes it easier to grab her later on.
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