Orphan's first form is one of the most frustrating bosses in the game, if not the entire Final Fantasy series. It has a huge number of attacks, two of which in particular are simply not fair.
Orphan will regularly switch between Consummate Light and Consummate Darkness, and each time it does (including at the start of the fight), it will use Merciless Judgement, an attack which reduces the party's HP to miniscule levels. This attack isn't a big deal by itself, though you can't paradigm shift during the animation so you may want to bring up a healing paradigm as soon as you see either Consummate ability being readied. Heal quickly, because
Orphan won't hesitate to finish you off if you give him a few seconds. Also note that switching forms resets
Orphan's chain gauge, so you'll need to act quickly to stagger him and get damage through.
The biggest dangers are
Orphan's ability to just kill your main
Character if he feels like it. Once low on HP, he will start using Progenitorial Wrath, a
Death attack that has a very high rate of success. If he uses it on your main
Character (and it works), it's game over.
Cherub's Crowns can help prevent this, as can making sure your party leader is not the party
Sentinel. Far more dangerous is the fact that
Orphan can
Poison you, even immediately before Merciless Judgement, and if you're poisoned during that attack, you die. There is absolutely no way around it, as you die during the animation while your party can't do anything.
Poison resistance could work, but you only have three
Accessory slots. Manually heal
Poison with an
Antidote or even
Dispelga if you have to, use
Veil, and just hope for the best.
Other than those considerations, this is a pretty straightforward fight. In one form,
Orphan sticks mostly to basic attacks, while in the other it tends to inflict a ton of
Status Ailments. Be ready to counter just about anything.