Assuming you're pretty well maxed on the crystarium, here's a chunk from the guide I used when I 100%ed the game (twice, once each on 360 and PS3):
Battle 21: Barthandelus (BOSS). This guy is a walk in the park compared to the last time you fought him, mostly because you're so much stronger. I just used the lazy approach with Fang, Hope and Lightning, and I still beat the target time with 2 minutes or so to spare. If you want to go faster, start in SAB/SYN/MED and stick him with DeShell and DeProtect. Then, COM/RAV/RAV to stagger, COM/RAV/COM to attack, and COM/MED/MED to heal. He only has one attack that you should be scared of: Thanatosian Laughter, which will take all 3 characters down by about half. Fortunately, the charge time for this is more than enough to switch to your healers and absorb the damage. If he takes his debuffs off, go back to SAB/SYN/MED and put them back on, and you'll be out of there with barely a scratch!