Can you describe a little more about what happens when your daughter acts out? When she gets violent or angry, does she end up getting what she wants? I had a very strong-willed son who would pitch big-time fits when he was little. I didn't give in to his demands, though, because I didn't want to teach him that he'd get his way with such unacceptable behavior. He could be very bossy with his peers, or, he'd act out because he simply wanted something -- juice, a toy, etc. He never seemed happy unless he was the ringleader However, he didn't learn that lesson easily. Even though he didn't get what he wanted, he'd pitch that fit on and on and on! Ugh! I really hated that time in his life! But I never gave in! It was no fun, because there were times when I literally had to pick him up and remove him from places that I really wanted to be -- parties, visits with friends, even grocery shopping. Hey! When you have only a short amount of time to get those errands done, it just seems like it would be easier to give in and hand the kid whatever he wants just so you can get through the checkout line! But, there were a few times when I just left the cart at the service counter, told them I was sorry, picked up my son and just left. That was the negative feedback he got. But, he always got lots of positive feedback when he did things the right way and played nicely, asked for things politely, etc. When he was going through that period, I tried to keep his play dates short. He could handle 15 minutes at a playground with his friends, but an hour at someone's house, and he was Mr. Bossy again.
I think that was the long hard lesson. He used to have his little meltdowns because he wanted something. I wouldn't give it to him when he did that. It was no fun, and it did indeed turn into something of a power struggle, but it was an important struggle, all the same. I didn't want him to learn to be bratty and demanding. Today he's 18, and what used to be bossy behavior has matured into a natural leadership ability. But, Lord forgive me for the times I just wanted to clobber that kid! :)
I don't know if any of this applies to your daughter. Is this new behavior? Maybe she's seen some friends her age doing the same and getting what they want. Or she's just testing her limits. Good luck! Stay calm, patient and consistent.