The second time you fight Barthandelus is in the town of Oerba, on Gran Pulse. Before the fight, make sure to have a party consisting of Lightning, Fang and Hope. Lightning should be far in her role as a Ravager, Fang as a Commando and Hope as a Medic. Set up these Paradigms:
Relentless Assault (RAV-COM-RAV) Combat Clinic (MED-SEN-MED) Soldarity (COM-SEN-MED) Delta Attack (COM-SEN-RAV) Mystic Tower (RAV-SEN-RAV) Evened Odds (MED-SAB-SYN)
The start of the fight against him is easy so go straight on the offensive using Relentless Assault until you need to heal then swap to Solidarity. Once he starts to change his form, swap to Evened Odds and give him and you some debuffs/buffs. Once Fang and Hope are done, switch to Delta Attack and bring Barthandelus's chain gauge to about 40%, then swap to Mystic Tower to charge to its limit, stagerring him. You will have to repeat this a number of times. This first stage of the battle should be your most aggressive phase, since he cannot perform any devastating attacks.
Throughout the beginning of the fight, Barthandelus will spawn "heads" in front of his main body. After few more have appeared, he will reveal his main face, completing the first stage of the battle. Once he has fully transformed he will be able to use the "Thanatosian Laughter" attack which deals moderate damage to all members, so make sure every has above 50% HP or shift to the Combat Clinic pardigm when he uses it. He also use a move called "Apoptosis" which does very low damage, however, it removes buffs on your party and any debuffs he may have on him. Other than this he can inflict a variety of status ailments, including Curse, which could mean a huge disadvantage, since he attacks relentlessly. When Barthandelus reaches about 500,000 he might cast Doom on your party leader. If he casts Doom, it's better to go full-out offensive and try to take out whatever health he has left, using Renew for healing instead of Combat Clinic. Alternatively, you can summon an Eidolon to halt the doom timer and grind away some of its' HP. Should you be under the curse ailment at this time, it spells an almost certain defeat, so heal it ASAP with the proper item.
After the fight you get a Goddess's Favor, 100,000 CP, and a Crystarium expansion.
Source: Final Fantasy Wiki http://finalfantasy.wikia.com/wiki/Barthandelus_%28Boss%29