There are 300 optional missions in Crisis Core Final Fantasy VII: Reunion. Secret in the Caverns (M6-6-3) can be found in the Seeking Precious Items Category.
How to Unlock Mission 6-6-3
Secret in the Caverns is a mission that will become available as you complete previous missions. In fact, you will only have to progress the main story to Chapter 2 to unlock Mission 6-1-1. Do this string of missions to eventually unlock Mission 6-3-1 (Missions 6-2-1 through 6-2-6 will be unlocked later). From there, you simply need to just do the missions before you to keep going, all the way up to Mission 6-6-3. Note that the missions will become progressively more difficult, so you will need to make sure you have good enough Materia and Accessories to survive.
Farming SP and Dark Matters in Mission 6-6-3
This mission isn’t all too challenging, since you won’t face a super strong enemy or anything of the sort. In fact, Secret in the Caverns is a primo farming spot in Crisis Core Reunion, for a few reasons. Before getting started, you will likely want to make sure you have two key Accessories, the and the . The former makes sure that enemies’ steals and drops are always their rare items, while the latter will ensure you always have a 100% steal rate. If you want to get even more out of this, you can also equip the accessory to double the drops from the enemies.
So, you’ll go through this mission and encounter three enemies in the normal battles, which are the , , and . The Horned Devil is probably the least profitable, but they do drop the Materia, which you can use to turn into SP. You will want to be a little careful around them, too, as they know Energy and Ultima, both of which are powerful spells. The Lesser Worm is one of the best options for farming, as they drop . Since they are so easy to defeat, you can amass a bunch of these and use them in Materia Fusion.
One little trick you can do is put a bunch of Dark Matters in a fusion attempt, up to one less than the max, then use another item to turn that Materia into another stat. These enemies can inflict Stop on you, plus they can cut into your MP and SP with their other attacks. Note that the final battle in this mission will be against a bunch of Lesser Worms. The last enemy on this list is the Mover EX. If you’ve encountered these types of enemies before, then you know they give you a lot of SP just for defeating them.
The base SP yielded by this enemy is 8,000 SP, but that can be higher if you don’t get hit and kill other enemies in the same battle first. For example, you have a Horned Devil in the same battle and defeat that enemy first, then don’t get hit and defeat the Mover EX second, which will make it yield 16,000 SP instead of the base amount. In addition to this, it can drop , which you can just sell for Gil, and you can steal the Materia from it to get even more SP. The Mover EX does have access to Graviga, as well as Delta Attack and Ultima.
As you can see from all of the info above, you can get a lot of goodies from this mission, making it a prime spot for farming SP, items for fusion, and even some Gil.