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Did you try using GameFaqs?

Well, then go back to the previous area, and level up a little. Unless you don't level up in XIII,... It's been a while.
On a side note, Hooooooooope. sweatdrop I swear that kid's turing me shota.

Okay, I've beat the game 3 times, so I'll tell you what I did; (and yes I'm a walking talking FF Cheat book. Ask me questions)

First, I maxed out the current ring levels for all SIX of my character, not just the three that I'll be using to beat Bartadelus.

Next, I had these three people in my party: Sazh, Lightning, Hope.

Now, Sazh's main paradigm roles should be Synergist and Ravager, seeings how your going to want him to buff you up and quickly. I have Sazh as my main in this level (unless like on my third round, you for some reason cannot change your main person) this way I can just bust out buffs instead of having him do them one or two at a time and go all three characters instead of one at a time.

Hopes main roles will be Medic and Ravager. You should make sure that he is almost always a medic (I'll tell you paradigms in a minute)

Lightning is going to be 3 roles; Commando, Ravager and medic, in that order of priority.

Now, your paradigms. They'll be Lightning, Sazh and Hope in that order just incase you can't switch your party leader.

Com, Syn, Med
Com, Syn, Rav
Com, Rav, Rav,
Com, Rav, Med
Med, Rav, Med
Rav, Rav, Rav

And here's how you use them:

You ALWAYS start off with your Com, Syn, Med combo. Lightning should be aiming for the rooks, and not Bartandelus. Your objective in the first part of the fight is to get them out of the battle before they have upgraded to their highest magic. If it gets that far, you'll find that you have a hard time getting to the main battle without losing most of your HP. Do not switch your Paradigm unless you need some extra back up. Hope will heal you whenever you get hurt, seeings how all the rooks will be after you, since yoru doing damage. Sazh will buff you with an elemental buff when he realizes who your after (and its better to use Libra before you start attacking, so he can do this buff right after casting Haste on you) You'll want to occasionally switch to the paradigm that has Hope as a Rav, so that you can get some extra damage in there, plus he should know some magic that'll be super effective against them.

Now, if Sazh finishes buffing everyone before the rooks are gone, switch to Relentless Assault (Com, Rav, Rav) Your ravs will follow your commando, so make sure your still going for the rooks. When the rooks are gone, quickly switch to your Med, Rav, Med (or you can do Med, Com, Med if you want to keep your chain gauge) your going to need to heal up quickly. Then switch to Com, Syn, Rav. Your objective here is to get him as close to stagger as possible, incase he desides to do Distructo, which will kill all your party members if your not lucky. I'll tell you how to beat that in a moment incase it comes to that.

When he does the Laughter move, make sure you have a Med on you and watch your MAINS HP. If it starts getting low, switch to Med, Rav/Com, Med and heal up. This is pain stakingly slow, but you'll get the battle done in around 10 minutes.

Now, if he uses Distructo, switch to Relentless assault (Com, Rav, Rav) and keep hammering at him DO NOT HEAL. You can also switch to Tri Disaster (Rav x3) to help boost the chain gauge but make sure you've had a commando hammering away at him, otherwise it will fall faster than you can keep it up. If he staggers, he will do very little damage, but the closer to stagger he gets, the better chance you have at surviving! If your lucky, he'll only do this 3 times to you. Either way, right before he goes to release the attack, switch to Med, Rav/Com, Med, in order to heal up your party before you go back on the offence.