The Mysidian Cave is located southeast of Mysidia, but to reach it, you'll need to go around a large circular mountain range. The quickest path is to head northeast from Mysidia, following the mountains east until they end, then turn back west. When the southern mountains end, head south and you'll see the entrance to the Mysidian Cave. The Monsters you'll meet during the trip are the same as those in the Mysidia Area.
Mysidian Cave B1
The encounters on B1 are similar to those outside, with a few wrinkles. You won't see Bombs or Behemoths, but you will see many more Malboros. The rare encounter combines four Monsters with very different weaknesses into one annoying fight. Red Mousses and Yellow Jellies will require Spells to take out, and are vulnerable to Lightning and Ice/Fire, respectively. Only Fire will work on both, so if you can defeat Red Mousses with your Fire Spell, you can save some time here, but that's a tall order. These slimes are joined by Red Souls and Yellow Souls. both of these are primarily vulnerable to physical attacks, with Yellow Souls going down easily due to their 20 HP. Their only attack is Fire 5, so they should pose little threat. The Red Souls, on the other hand, sport decent Defense and 540 HP. Both color souls absorb all elements, but you should now have access to Holy, which deals non-elemental damage. Level it up enough, and it will be extremely useful in encounters like this one.
The layout of B1 can be frustrating to navigate. There is essentially a narrow hallway branching out all over the floor, with various short staircases leading to clearings. You'll have to head west from the entrance. Further west you'll find a door leading to a trap room and a chest with a rare Gold Hairpin, a fantastic piece of Armor for any mage (especially one with low Evasion). Heading south instead of west leads to a four-way intersection. The small area to the southeast contains Black Garb, a versatile piece of armor that even mages can make good use of. The path further southeast leads to another trap room. The third and final trap room on the floor is west of the Black Garb, but heading that way is the quickest route to the stairs to B2, which are found behind a door in the southwestern part of the floor. A doppelganger awaits you there, and you must use the Black Mask on it in order to pass and continue downward.
Mysidian Cave B2
You'll find fewer Black Flans (and no Red Mousses with them) on B2, but more Undead and Malboros to compensate. In fact, the new rare encounter is with a group of 1–5 Malboros, which can be difficult to deal with if your front line fighters lack sufficient Evasion to avoid Paralysis. The various Monster groups in the Mysidian Cave will eat through your MP, so be careful. The all-powerful Osmose Spell awaits you on B5, but you have to make it there first.
B2 has a similar style of layout to B1. Once again, you'll climb some stairs to a narrow pathway that leads between areas. The stairs to B3 are actually quite close, found just to the southwest of the starting point in the corner of the floor. You can find a Flame Lance and Power Staff if you explore, though. (Both are available for sale in Mysidia, so you may want to skip them. You can always come back later with Drain and Osmose, which will make the journey a lot easier.)
Once again, all the stairs except for the ones leading to the next level lead to trap rooms. There are four of them in total. The northwest area holds nothing of value, but the clearing north of the entrance holds the Power Staff. You can see the chest with the Flame Lance in a clearing in the northeast corner, but there's no visible entrance. You can walk through the wall on the southern end of this clearing, as if continuing from the east-west hallway nearby. (Don't think too much about the implied height difference here!) Two more clearings with doors to trap rooms can be found south of that point. The hall between them will take you back to the entrance as well as the stairs to B3.
Mysidian Cave B3
You'll encounter several nasty new Monsters on B3, replacing Black Flans and Cockatrices. Malboros are now by far the most common Monster, showing up in various different types of groups. Flying Rays will now appear, offering another 750 HP threat with a lot of attacks. The nastiest encounter is fortunately the rarest, the same group of up to four Rhyoses that you last saw guarding the Orichalcum in the Castle Fynn Basement. Watch out for Stone from Bad Breath, and chip away at their 750 HP by whatever means you have available.
The good news is that the layout of B3 is simpler than the floors above. There are only two doors, and they're both in the same area. Once again, there are two chests. The one in the northeast corner once again seems inaccessible, but can be reached by passing through a false wall. This wall is found two steps above the east-west hallway leading to the area. You'll find an Ice Bow inside. The second chest, as well as both doors, can be accessed via stairs in the southeast corner of the floor. Follow the path east when possible, south when necessary, to reach it. (If you skip the Ice Bow, looping around north, east, then south from the stairs back to B2 is a more direct route.)
This southern area spans the entire floor. The chest contains an Ogrekiller, guarded by a group of Bombs. They should pose a minimal threat if your Characters' Defense values are reasonably high. You'll find a door to a trap room further west, after which the path turns north. At the end of it, you'll find the door hiding the stairs down to B4.
Mysidian Cave B4
There is only one new encounter on B4, a Rhyos or two accompanied by Death Flowers and/or a Ghost. All of these Monsters are individually dangerous, and as a group their sheer numbers could prove a problem. The Ghost, if it appears, can make it harder to kill everything else quickly, so casting Fire on it should probably be your first priority. You'll find no more Brains on this floor, but the slime/soul encounter moves into a common slot.
This floor has a very simple layout which will require a lot of walking. You'll have to venture all the way west and south before heading back east and finding some stairs to the central path. The eastern branch is a dead end, so start off going west and follow the path to the stairs to a large eastern area. You'll find an easily accessible chest with a Potion before the area splits in two. To the west you'll find three doors, while the door to the southwest leads to B5.
The central door leads to a Treasure Room with a nastier encounter table than even the trap rooms in the dungeon. Rhyos groups in various configurations are very common, and you'll see some encounters not otherwise found in the Mysidian Cave, like combinations of Flying Rays, Gigantoads, and Wild Horns, or Ghosts and Death Flowers. Don't spend more time in here than you have to! You'll find a Gold Needle, a Phoenix Down, and the unique (but unimpressive) Bell of Silence within. It's a single-use Silence 16 which will likely work on any Monster not resistant to Body, but whose Mute effect can wear off at any time.
Mysidian Cave B5
The encounters on B5 are the same ones that appeared in the B4 Treasure Room, except the most difficult encounters are the rarest rather than the most common. You'll still have to deal with Rhyoses in one out of every five encounters or so, but generally you'll be fighting the same sorts of things you've seen so far.
This is another simple floor with a very long walk. There is a Drain Tome near the stairs to B4, but it is guarded by 2–5 Ghosts. This can be a difficult fight, but the prize is well worth it. Drain does as much damage as the Elemental attack Spells, but has no Element and therefore isn't resisted by anything… with the notable exception of Undead. Drain will restore as much HP to the caster as the damage it deals, but against Undead the effect will be reversed. This is a Spell worth leveling, and your first non-Elemental black magic Spell.
You'll find stairs to another path to your southeast, and that path immediately splits in two. The western branch leads to your goal, but the eastern path hides the entrance to the apparently inaccessible area to the north. With five chests, this may seem like a worthwhile endeavor, but the southern chest contains a Mini Tome you can buy in Mysidia, and the northern chests each contain a Potion. The entrance to this area is in the middle of its eastern wall.
Whether you grab the disappointing Treasure or not, you'll need to next head as far west as you can go, and then south. You'll reach another set of stairs, and then the path hooks back north, leading to a door. The door is unguarded, and there's no boss waiting inside. You'll find the Crystal Rod you came for, a second Drain Tome, and the real prize of the Mysidian Cave: the Osmose Tome.
It almost cannot be overstated how good Osmose is in FFII. It works exactly like Drain, including the reverse effect against Undead, and even deals as much damage as Drain does, albeit to MP rather than HP. The caster will gain the same amount of MP. What takes Osmose from great to broken is that it can drain more MP than the target has, even draining MP from Monsters who had 0 MP to start with! It always restores at least 10 MP per Spell level, so as long as you have enough MP to cast Osmose, your black mage will be able to get more. Worrying about MP for black magic is a thing of the past, and with the Swap Spell available and some clever planning, all your mages should be set going forward.
Once you have the Crystal Rod, you can leave at any time. You'll need to use Teleport or Warp to avoid having to walk out.