I am at what I assume is the last boss fight of the game, I have deafeted Darth Vader and am fighting the Emperor. I have him down to where his health meter is nearly empty. This generates a finishing move sequence of about 8 or 9 shapes. I hit them and then at the final move the last prompt I hit the appropriate shape but he still hits me with a lightning bolt and then which replenishes a small amount of his health and then I do the whole thing over again. I have done this about 5 times. Is it just a matter of hitting the last button quick enough because I am hitting it as fast as I can and it is the right button?

yes, try not playing for just a bit, because playing a long time can make your hands slow, know what i mean, happens to me sometimes.