The second to last fight in the main story line was easily the hardest fight on both of my playthroughs. Though it was significantly easier the second time around, it was still the hardest fight of that playthrough.
If you do not build up the chain fast enough, he will launch an attack that decimates your party and resets his chain gauge at the same time. I have even had him do this in the middle of a stagger, and it seems unpredictable when he is going to do this, so timing your stagger just right is impossible. Then he hits you with a toxic cloud that afflicts your entire party with status ailments, some of which are extremely dangerous if you do not clear them immediately. And there is a looming threat throughout the entire fight that he will insta-kill one of your party members; you have a 33% chance that this will immediately end your fight.
You have to be fast with this fight, even faster with the one that follows
That said, if you can perfect dealing damage at full throttle, you can easily finish the second fight so quickly that you will get 5 stars. If you do this, you will win a Gold Trophy (PS3) or 60 Gamerscore (360).