Remember where you got Fusoya earlier on the moon? Head back to the Crystal Palace, saving outside before you enter. Restore your HP / MP and go to the back of the moon - you are transported to the Lunar Subterrane B1.
Lunar Subterranean B1 : Notice you can walk through the side walls here. Head through the left wall and go south when you can to find a blue teleport. As you get out, head south.
Lunar Subterranean B2 : Go north and get the Sage's Staff but you have to fight a Dark Sage and two Moon Maiden's. Follow the path to another teleport, taking you to the west end of B2. You can walk west through the wall, or just go south for the exit.
Lunar Subterranean B3 : Head north to see a thin sword being shown off. You will be fighting a White Dragon here to win the sword however. It has powerful physical attacks and abilities to hit the entire party. Watch out for when it uses Slow and follows up with Maelstrom - it is trying to screw your healer Rosa but if you are patient enough and get Haste back up, you should be fine. The Murasame for Edge is your reward.
Back to Lunar Subterranean B1.
Lunar Subterranean B1 : Walk through the east wall heading northeast, and follow the long path as it wraps around the exterior of B1. You will come to a Black Garb but you have to fight an Armor Construct to attain it. This is the best armor for Edge. The Armor Construct enemy will paralyze you and has some powerful physical attacks, but should not provide the challenge of a boss certainly.
The south path in Lunar Subterranean B1 will complete the map for you. 10 Hi Potions are your reward.
Lunar Subterranean B2 : South and east to walk through the wall. Work your way north a bit and then west to a treasure that is the Flame Whip. Before returning back, head all the way to the northeast and you can come out in this new area. Keep heading west to complete the map for 5 Ethers. Far to the northwest here is a Dragon Shield. Take the northern exit to B3.
Lunar Subterranean B3 : You start off in the north end of B3. It is a huge area, but there is not much to do besides find the staircase going down in the center. But go all the way to the south, and try to walk east through the wall. You will come out by a Dragon Helm. Keep searching to the east and you can find another hidden path that travels south, and you come out to the west to get the Dragon Mail. To the west is another chest (Dragon Gloves) and you can complete the map, for 5 Ethers.
Lunar Subterranean B4 : Go to the far east as you get there to get Artemis Arrows. Back to the staircase going down, and take the left path. Go inside the cave for an Elixir, and return outside and go to the far east for the stairs. Follow them to a side passage, where you can find a White Fang and X Potion. Take the west door to end up at the southwest part of B4. Grab the Stardust Rod but you'll have to fight a Behemoth for it. Complete B4 map for 3 Dry Ethers.
Lunar Subterranean B5 : Head down the bridge and west all the way for a Crystal Shield. You will have to fight two Ice Dragons for it. Continue on through the door, and go all the way north and east through the wall for a Protect Ring. Make your way through to the east end of the map, where there are two exits. The south exit leads to Crystal Mail but you have to fight a Behemoth for it. Then take the center one with the stairs going down, back to B5. Grab the Crystal Gloves to the east but you will have a dangerous Red Dragon waiting for you. They like to use Thermal Rays, taking off 2500-3500 on each member, and they spam the ability. Your only hope is to use Slow and then Haste your party, and whittle his HP down. Or cast Berserk on him so he just attacks Cecil instead of using Thermal Rays. When Berserk wears off you will have to use it again though.
Take the west path and go in the cave. There is a treasure in this cave, White Robe, and the exit in the middle. The path south leads you to the next area. Before you go this route you can go west for a Crystal Helm, but you will have a Dinozombie to defeat. Not a tough fight at all compared to those Dragons. Also grab the Red Fang in the cave to the east.
Lunar Subterranean B6 : You start off in the north part of the screen. Go west and east for a Fuma Shuriken on each side. Then head due south veering to the west as you go south for a Cottage. Before you take the southern passage, look west of the Cottage as you can walk across an invisible bridge. This takes you by a Golden Apple, and another teleport to the east that takes you to the far west part of the screen. You can take the northwest exit to complete for map for an Elixir. This exit takes you back to B5.
As you head north there will be a Protect Ring, that is guarded by a Behemoth. A Protect Ring is a perfect accessory for Edge, and I have one on Rosa too. Complete the B5 map by going to the northwest and you will acquire 5 Sirens. Take the teleport and then save.
In this area, go east to see a sword floating. Watch out, this is one of the toughest battles in the game as you are up against Dark Bahamut. He hits you with Mega Flare to start the battle, and you have to hope you are in the mid to high 60's to survive. Chances are at least Cecil, Rosa and Kain will be alive. If Kain dies though and you have Phoenix, then you have your party revived. Dark Bahamut will quickly start using Flare on you and then Reflect on himself. You will want to Haste your party and get him Slowed, but you might have to Dispel him and then get Slow on him before he casts Reflect, or just cast Reflect on a party member and Slow on said party member. Dark Bahamut still moves fast even when slowed. If you attack him he will counterattack (not all the time), but if you have Blink up you will dodge it (Edge dodges attacks regularly too). Towards the end of his 60,000+ HP, he will start casting Heal for 4000-4500 HP restored each time. This will prolong the inevitable but you should be able to defeat him once you get Hasted and everyone back alive after the first Mega Flare. Your reward is Cecil's ultimate weapon, the Ragnarok. Be sure to head to the Save Point and save after this battle.
Back to B6 and take that exit southeast of the invisible bridge. There is a side passage on the way to Lunar Subterranean B7. Grab the Minerva Bustier but you will have to fight a Dark Sage and two Moonmaiden for it. Take the exit for B7.
Lunar Subterranean B7 : Head southeast and enter the first cave. You have a hard-earned save point here. This is a useful place to be when you want to fight a hard treasure boss or one of the greater enemies guarding the special weapons / armor. Also in this room is a Cottage.
The first door east of this has Plague Horror guarding a powerful lance. Was not impressed with this battle. Just attack with Kain, Cecil and Edge. If you have Ragnarok, this will be that much easier. With Rosa you will want to use Holy and Bahamut with Rydia. No need to Haste since Plague Horror does it anyway. It will use Protect and Shell so you may wish to use a Dispel spell instead of Holy for Rosa. Or just keep going at it, you're likely to take it down with 2 or 3 seconds left even without perfect augments. The Holy Lance is now yours, Kain's best weapon.
One more door east and you have another special boss, the Lunasaur. His Bad Breath attack will screw your ailments up if you are not prepared or are unlucky. Cecil routed my party one time. If you can survive that, you should be in the clear. Lunasaur mainly casts Bio on himself after using Reflect, or he'll do a physical attack. If you have the Ragnarok you will maul this Lunasaur with your Holy attacks. Kain does good damage as well and having him jump to avoid the initial Bad Breath is a good idea. Take the two Ribbons as your reward, as you can guess they work well with Rydia and Rosa. They prevent status ailments. Save and travel to the north part of B7. Complete the map for 3 Coeurl Whiskers and you can continue onward.
Lunar Subterranean B8 : West is a dead end, so go east and grab the Fuma Shuriken as you work your way back west. You will fight Evil Masks on this floor. They like to cast Reflect on themselves and your entire party, and then will proceed to use Holy and Flare on himself back onto you for high damage. Try to cast Haste right away on your party after he uses Reflect on himself, and then use Slow on yourself after he puts up a Wall on your entire party. Go all the way south to clear the B8 map, netting you 3 Gold Hourglasses.
Notice there is another sword on the southern podium, the Masamune. Ogopogo is guarding it, a fish that looks very similar to Leviathan. Not a terribly difficult fight, as Deluge should only put Rydia in danger especially if you can get the front row guarding. It will use the ability twice to start the fight, then will do some physical attacks. Heal up after this, and then cast Slow on him. This should help immensely as usual, along with Haste on yourself. Everyone else should be attacking and Rydia using Bahamut, as he does not counter physical attacks at all and only uses a weaker blaze attack to counter magic.
Lunar Subterranean B9 : Travel south, taking the east path for an Elixir. Follow the path west and back south for a Gold Hourglass. 3 Light Curtains for maxing the B9 map.
Lunar Subterranean B10 : Another Fuma Shuriken as you work your way south. An Elixir to the west and for completing the B10 map you get 3 Stardusts.
Lunar Subterranean B11 : New enemies here, Zemus's Breath and Zemus's Malice. Both are pretty weak enemies and Zemus's Breath likes to use Libra on you. 3 Lunar Curtains are your reward for finishing this area, and this is the last area before the end of the game. If you want to go back and save, do it now.
Zemus is here, with Fusoya and Golbez. They'll use a Twin Meteor on him to destroy him, but he has another form. They are not as lucky with this one, as Golbez's dark past makes the Crystal useless when he uses it on Zeromus.
Once you get in battle you can use the Crystal to release Zeromus' real form. This ugly creature will start off with Black Hole, which will negate any buffs you set on yourself and any negative ones on Zeromus. Next is the Big Bang attack, so try to get a Haste in right after the Black Hole and then you can easily guard with everyone to survive the Big Bang. When he rumbles, he is just about to use the Big Bang. This tactic uses Sap on your party in addition to doing heavy damage, so Cure up and maybe have Kain jump ahead of time to avoid it. Try to use Slow on Zeromus but when he uses Black Hole you'll have to do it again along with casting Haste again. As long as you can keep up and remember to defend before Big Bang you should be okay. Have Edge through weapons with the highest attack power like Fuma Shurikens. Cecil should attack, Kain should Jump, Rydia should defend, use Asura or Stardust Rod for a Comet attack. Rosa of course should be busy with Curaga / Curaja and getting buffs for your party and Slow on Zeromus.
Final Fantasy IV 4 DS walkthrough