he's actually a lot easier than we are making him out to be.
start of by getting some 'cheap' shots with the horizontal blade (L TRIGGER), like one or two, maybe three. This hurts him bad. You should be able to knock him down almost 1/4th right off the bat. After a few of these he will start to attack and stop transforming.
Pull back, STAY OUT OF HIS LASER CANNON RANGE, AVOID HIS VOLLEY SHOTS, AND DON'T LET HIM EVEN TOUCH YOU WITH HIS MULTI-ARM SWORDS. I used R TRIGGER the WHOLE time and just laid into him.
Avoiding all his attacks, not-stopping with R attacks, and once in a while use X (vulcan cannon), you should have quite a bit of health at the end. You will notice that his health is at 0%. At this point you must fly up to him and finish him off with TWO L TRIGGER attacks. Easy. Make sure you dont haphazardly just fly in at the end, get in between his attacks.