perhaps you already know this by now!
If your characters' levels at that moment sucks and can't even bring up a decent fight with the mobs outside, then I'm afraid you'll have to waste a lot of time training with those mobs, very hard on low levels! otherwise, restart the game from a a more further past save can help (hope you have multiple saves)! or just restart the whole game! this time, mind your ranking, try to get 5 stars for maximum CP! Save you a hell of frustration!
beating barthandelus is quiet easy if you level up as close as maximum allowed as you can! in battle, use relentless assault (COM-RAV-RAV) or agression (COM-RAV-COM). Take out each head off as quickly as you can because he'll use magic amplifier soon, then he'll wail every element-ga forms on you, it's pretty much game over!
once you're done with the heads! sometimes he'll use Destrudo(don't remember the spelling!). forget healing! full assault him to shudder him once! you can't cancel that attack by attacking though! so once he shudders, better get to healing and defensive paradigms to minimize the damange, then heal! then lauch some physical-based attack of COM on him! switch to tri-disaster to stagger him quickly! drive the bar up for while but keep it still nearly full (mean it'll be long until he recover from staggering)! if he's low on health (2-3/10 health bar), summon Odin and Spam Left-Right-O on him but it's better to synthesize with other attacks! keep an eye on the gestalt points and the stagger bar! when points run out, or the stagger bar is about to recover, use Odin's ultimate attack! if you're strong enough, the blow will likely finish him! otherwise, a little push will do!
oh if he casts Doom on you, using an elixir to recover TP to max then summon ODin again!
Barthandelus gets easier later fights! Sorry for the wordy, that's pretty much of my strategy to beat him first time! there's no way you can beat him on low level without a hell of hardship! no matter how skilled you are on low level, your attacks still sucks! at least training a bit make things easier!
[Edited by hayden24, 4/12/2012 6:08:04 PM]