I'm walking in with Lightning (COM), Vanille (RAV/SAB/MED) and Snow (RAV/SEN) and am simply getting pummeled, especially when Orphan comes out of stagger. How do I keep myself from grinding for hours to finish this game? Is it too late (or even necessary) at this point to backtrack to Gran Pulse for some character development?
First of all, the FAQ of IceQueenZer0 and Split Infinity from gamefaqs.com is quite helpful. I found most of my information there.
Concidering Orphans first form, this is to a large extend a matter of staying alive. Be prepared to always heal and debuff the party completely, since a weakened party can be wiped out by a single attack (so you should bring MED/MED/SEN or even MED/MED/MED). To bring down Orphan, I found poison very helpful. In fact, when I was fighting Orphan, it perished almost unoticed by me due to the poison while I was panicking to keep the party alive. Also, I found it almost impossible to stagger Orphan since its stagger bar was constantly resetting.
On the other hand, its second form was much easier. While it seems invulnerable in normal state, once staggered, it takes heavy damage. The key is to hex Orphan while building up stagger as fast as possible, and then hit hard. Rinse and repeat, and you are done.
With respect to your question about leveling on Gran Pulse, you can always leave the Narthex through the portal behind you which will send you to the portals in the Cradle. From there, you may move freely between Gran Pulse, Eden, or the Narthex. However, I didn't find it necessary to level up at this point of the game. (Your Crystarium should already be almost maxed out on the Characters core disciplines anyway.) Success in FF XIII is much more a question of the right strategy.
Orphan First Form
The biggest challenge in this form is keeping your hit points at almost maximum, while dealing enough damage to defeat Orphan. The Merciless Judgement attack will almost completely deplete your hit points and there is no warning before it occurs. From my observations, the attack always occurred near the end of the stagger period, but obviously that timing depends on how fast you stagger Orphan.
I used Fang, Lightning and Hope, which meant that I was unable to Poison him, but I still managed to squeak through with a victory on my fourth attempt. My basic paradigms were SAB/MED/MED to heal quickly and debuff him, then RAV/RAV/MED to stagger while healing and COM/COM/MED or COM/COM/SYN while staggered, switching to SAB/MED/MED near the end of the stagger gauge (usually after using Fang's Highwind attack to do a lot of damage and clear the stagger gauge). I guess it is important to note that I did spend CP on giving Fang level 1 in RAV, so that I could raise the chain gauge faster.
As far as items are concerned, Fang had a maxed out item that protected her from Death, all weapons were maxed at tier 2 and everyone had a maxed out hit point bangle. I also went into the battle with maxed out TP, so that I could use Renew twice to quickly heal everyone if an attack left me extremely vulnerable.
The biggest problem I had in the battle was keeping everyone's hit points high enough to survive the Merciless Judgement attack during the time that the black ball was active. (Orphan has two "modes" - a black ball that debuffs and attacks you and a white ball that heals him - he switches between the modes after each Merciless Judgement attack.) The problem with the black ball mode is that he can Daze/Fog your characters, making them stop healing for a portion of time, while dealing large amounts of damage. I managed to survive through quick item use and through favouring SAB/MED/MED during the black ball mode, even when Orphan was staggered. (In other words, keeping your hit points up is more important than dealing damage - there is time to deal damage during the white ball mode.)
Orphan Second Form
This battle was actually quite easy. He is invulnerable except when staggered, but his attacks don't do much damage, so a simple RAV/RAV/RAV and then COM/COM/RAV combination works extremely well.