In Giant Kill mode, Kai with the Pheonix Talons is the best. When you
start the battle you are most likely to be the giant if playing offline.
As soon as you start, wait until everyone encircles you and press Y
repeatedly. After you mass about 10 kills, it seems like there will be
much more than usual, but there is not. They will become smarter and try
to back you into a corner, which is very annoying. At this point, pick
up the hammer and repeatedly press Y. Because you are the giant, you are
more powerful than a regular fighter. About three swings should kill a
group. Do not worry about health; you have a lot.
