Tips before the final bosses (as you enter the cradle for the first time)~
If you havent maxed out your 3 base classes, then it might be a good idea to do so (as far as possible of course, level 10 only opens after you defeat the final bosses).
I suggest upgrading weapons onto 2nd tier upto their max level (if not already done so) and accessories if you have any money left.
One way to get money is to fight the human enemies in the city before you reach cradle (which can also be reached from a teleport inside cradle) or go hunting in the pulse wilds for gold nuggets or gold powder.
As you complete cradle you'll have a lot of annoying enemies on the way, so be prepared for some tough long-ish fights. It depends a lot on your stats so if you want it easy then grind those classes!
A few tips for the final bosses~
The team you use really doesn't matter, but this is what I had (only stating classes used):
Lightning with Com 4, Rav 4, Med 4, Sab 1
Snow with Sen 4, Rav 4
Hope with Syn 4, Med 4, Rav 4
I had the following paradigms:
I'd start out with Strategic warfare, give a few blows the time hope boosts everyone with haste, then I'd switch to premeditation, let hope boost everyone with other stat enhancers, while snow takes all the damage and lightning casting poison (yes, strange how the final bosses can get poisoned! XD). But poison really helped me out since it dug into the boss HP quite well. Beware that inflicting debuffs is difficult as the chances are very slim. As soon as I poisoned (and maybe cast other debuffs with luck) I switched between combat clinic and tri-disaster, keeping my HP high and increasing the stagger gauge. Once the stagger gauge popped I switched to relentless assault to inflict a max amount of damage.
I used this technique for all 3 bosses at the end, so you technically don't really need to change anything, just keep up the same strategy and you should be fine!
As Mazrael pointed out in his comment, there is one attack that orphan's first form uses which has a risk of OHKO. The damage and who it attacks is random, but if you're close to beating him, try to stagger and use the eidelon (or just simply use the eidelon if you're comfortable with their damage). You won't need the summon for orphan's last form, as it's pretty straight forward.