There are 300 optional missions in Crisis Core Final Fantasy VII: Reunion. The Desert Island Surprises (M6-3-2) mission can be found in the Seeking Precious Items Category.
How to Unlock Mission 6-3-2
The Desert Island Surprises mission can be found under the Zack, the Treasure Hunter sub-category that you can unlock by completing all of the Looking for Items (M6-1) sub-category that first unlocks in Chapter 2. You must then complete the first mission in the Zack, the Treasure Hunter (M6-2) sub-category to unlock this mission.
How to Defeat the Pachyornis
Most of the missions in this sub-category are relatively straight forward and most players will complete them as they encounter them at the start of Chapter 2. This particular mission, however, becomes a bit more challenging when you reach the Pachyornis at the end of it.
Before you head to the boss in the southeast, take the time to open the three chests to obtain a Hi-Potion, Bronze Armlet, and a Mag Up+ Materia. You may wish to consider ensuring you have Stun protection before proceeding to the battle too, as the Pachyornis is especially fond of the status effect. Accessories such as the Headband, Aegis Armlet, and all of the Chocobo Armlet series of accessories will protect against this.
Once you start the battle, you’ll notice that the Pachyornis is accompanied by another, and that the battle arena isn’t the largest. It is, then, a good idea to bait the Pachyornis into attacking by running around and then going in for the attack.
The Pachyornis (and all other enemies of this class) have two major attacks. The first is a simple peck combo that will make Zack flinch and deal a reasonable amount of damage. However, it’s the second attack, that is where the difficulty comes with this mission.
There is no ability call-out for this attack, and it looks no different to a regular peck. This, however, is one single peck that will deal heavy damage, knock Zack to the ground and inflict Stun. This is especially deadly as it leaves you completely open to being attacked by the other Pachyornis who may even perform the same Peck attack on you. This is where the Stun resistance comes in handy, otherwise you will find yourself having to repeat this battle a few times.
As with most Missions in Crisis Core Final Fantasy VII: Reunion, the more time you spend powering up Zack, the easier it will become so your experience may vary when it comes to encountering these beasts.