Its a phase. Just keep reinforcing that you don't like to get hit.
Well i guess because you do not want to go disciplinarian on her you better invest in some cream for the bruises. Because she is only going to get better at punching :D
You have to discipline her. She's your child. You can't reason with a six year old. She's challenging your authority and sounds like she's winning.
give her something else to punch....explain that it isnt nice to punch people....ask her if that is what the panda does in the movie
and thank you for best answer!