The entrance to the Void is located where Castle Tycoon was. Out in the middle of nowhere, a dot on the map, with a sort of black hole, is where you can access it. While once you get there, the dialogue will imply there is no way out, you can actually leave and come back however many times you want.
Whether you use a party of jobs you want to master, or one of Freelancers and Mimes, is entirely up to you. But please use Freelancers and Mimes for superbosses.
The first area looks a bit like the Desert of Shifting Sands. The path here is fairly linear. Simply follow the correct shifting sands to move across. Be careful, the enemies are really nothing to scoff at here. Rapid Fire is helpful, but not an instakill. The Landcrawler is especially hard, as Maelstrom can get you down to 1 HP. It is not too fast, however, and gives plenty of ABP.
Eventually you should come to a door. When you get there, you will learn that Exdeath's assassins are trying to kill you. Then, walk through it. You are now in a new area.
In the mechanical ruins, like Ronka first head downwards and then head to a door below. In this next room, you are confronted with another door. Pick up the treasures to the bottom right and bottom left first. Bottom right is a Cottage, left an Ether. Now go through the two center doors. To the left, go around the cog to get an Elixir. To the right, Dark Matter. Go through the door at the bottom.
Go right, go down the moving chain thingies, down, left, down more chains, now head right. You will come to three sets of moving chains. Central one is useless. Far right first, that will get you to an Elixir treasure. Far left takes you to a door. Now go up these stairs in a new room. You are now confronted with two more stairs. Go up the ones to the right and head up for a treasure, a Blood Sword. Go back, and go up the stairs to the left and through that door. Now go up the moving chains. and head down.
Important to note, there's some juicy Blue Magic in here. The Level Checker has loads, however I normally get them before fighting the Checkers.. You can also learn two other Blue Magics which I recommend learning here. One is fairly useless, but anyway:
That's not "kills a random enemy", it kills anyone on battle. So it's 1 MP, it MIGHT kill an enemy, or it might kill one of you. Worth the risk? Well, for 1 MP, possibly. Remember that a lot of the harder enemies are immune.
Learn it from Parenthope at Phoenix Tower, it's easy there.
The other is almost completely useless but will help us against a boss soon:
Non-elemental piercing damage to a target based on caster's HP, killing caster
What is the point, seriously? Like Transfusion, it just kills you to do some damage to an enemy. Bear in mind that your damage output scales much greater than your HP, regardless of the FF game. So you will never have so much HP that you'll deal more damage with Self-Destruct than with any attack. You'll also deal that damage in a more consistent and sustained way. There is literally no point. There is one boss that is made ridiculously easy using this method, and that's it.
You will probably learn this against any Bomb enemy in battle with a Blue Mage accidentally. But otherwise, Prototype is the best to learn it from.
You're back in Phantom Village now. Don't worry, nothing here that can hurt you. You cannot buy anything. What you can do is use a restoration pot for free that restores your HP and MP. Then you can head out to the exit of this village to the right. Talking to NPCs does nothing other than induce a "???" response.
Gulp, do we have to? Well, this place is more linear than Moore and the enemies are not quite as hard since you're more prepared now. The Moss Fungus enemies die to Level 3 Flares. First things first, White Flames teach sweet Blue Magic here. One is incredibly useful, the other is not.
Removes caster from battle permanently but fully restores a single target's MP and HP
Transfusion is a very useless Blue Magic and also very weak. Its ability, restoring a target's MP and HP completely, is made redundant by bother Elixir and the Phoenix summon. There is no reason to ever use this ability. Not even as a last resort, it's bad even in those cases as removing a character does more harm than good. It's fairly pointless.
When I played, I learnt it from Devilfish near Exdeath's Castle, however, that is unavailable in World Three. I would recommend learning from the Unknown. However, Mythril Dragons are also early in their appearance and easy to fight.
Restores party HP equal to the current HP of the caster
One of the historically much more useful Blue Magics, White Wind is situationally awesome. It's also learned from relatively easy to spot enemies. White Wind is a very powerful healing tool that, unlike spells like Cura, Curaga and the like, don't rely on any Magic stat or on the spell itself, only on that of the caster. This means the spell is always very useful. Consider picking White Wind up. The only problem, of course, is that it requires using Blue Magic to do it.
The Enchanted Fan, Dark Elemental and White Flame are the best enemies to learn this from. They have to be Controlled by a Beastmaster to actually learn it, as White Wind will never be used on the party otherwise.
Anyway; when you get in, head straight right and go upwards through a path. When you reach a tree, go to the left of it. Go around behind it, to the right and up, for a Dragon Fang. Go back, and this time, go top left from the tree. Head up and right. When you meet a dead end, go down and to the left a little is a Ribbon, to the bottom right is a Lilith Rod. Now head as far down (a bit to the left) as you can until you meet a tree, go around to the right of it for an Enchancer treasuure. Go back up a bit and to the right through a gap then down. Go further down into a tree, and interact with it, for it to open a path. Now get ready for an (easy) boss battle.
This is nothing compared to the superbosses we are about to face, so do not worry. Equipping Rapid Fire on those who have it is nice and makes it easy. That said, you should have White and Time Magic on you.
Magic is the best way to beat her. So Dualcasts of Bahamut and Meteor are the way to go. She will cast Reflect, however, so don't use Black Magic. Spellblades are not a horrible idea. While she is weak to nothing, she is not resistant to anything either.
Last one before a new page, I think. This is where we will fight a big bad Superboss. Likely, your MP was drained from last fight. Don't worry, we have a Save Point just before it. For the record, the Achelon (enemies who share Carbuncle's sprite) will actually Petrify you. To kill them and get a Bestiary entry, just cast Bahamut on them. No MP? Elixir. If you used an Elixir, Bahamut every enemy here, since there's a Save Point.
Head right and down the stairs, keep descending this linear path. Next room, follow the path right. When there's a cave entrance, ignore it and carry on right, go through the waterfall, get an Angel Ring, go back out. And now go through the cave. In the new room, take the steps up and get a Coral Ring (though there is little use for it if you already have all 4), then just go right and down the stairs to Save Point room.
Rapid Fire and Spellblade for Freelancers. On Mimes, Time Magic and White Magic on one, the other, Dualcast, Bahamut and backup White Magic. Black Magic is useless this fight, so don't equip that on the Mimes.
Equip everyone with Coral Rings. And equip two bladed weapons on Freelancers. Ribbons are very nice.
If you are not fighting Omega, ignore this.
Use Cottage when you switched, then head down. There's a robot walking in this next room. Get ready...
Everything else(absorbs EVERY element but Lightning)
Omega, or Omega Weapon, is a name of a type of superboss in almost every Final Fantasy game. And this game was where he notoriously made his debut. To fight this, you need to be very well prepared both in game and mentally, as this can take several tries. Even the most prepared of parties have difficulty. Do not be fooled by the normal battle music playing, this boss is way harder than a normal battle. Rapid Fire and Spellblade on two Freelancers, two blade weapons on each Freelancer, and Coral Rings are what you need.
The key is mostly to survive three turns. You need to Thundaga Blade on two Freelancers, then Rapid Fire with them. Your Mime should have White Magic and Time Magic. Hastega first, of course. If you have any spare turns, which is unlikely, Dualcast Meteor or Bahamut (neither do a whole lot). Otherwise, Curaga, Arise, anything and everything to save you from this guy. He hits hard. I would normally say "don't bother with Curaga and focus on Arise", but if you do that, you can easily be killed on low HP. Coral Rings on everyone to avoid Atomic Ray among other things. Oh, and Rocket Punch? It goes through Ribbon. If you lose both of your Dual-Wielding Spellblading Freelancers, you're toast.
Just keep calm and fight on. And excuse my ridiculous meme spouting. Omega in this game is really damn cool I must say, giant robots are always awesome. But he's also a huge pain to fight. It's down to luck, skill and preparation. The chances of everyone being alive when it is defeated are minimal. Oh, and when you beat it, you get an Omega Badge for brag rights. Mobile version gets a better brag right prize (an achievement).
If you aren't prepared, you can skip it by avoiding him. If you did, you might want to go back to the Save Point now. No, go to the room to the right. In here, open the book. You have to fight a boss but it's way easier.
Point and laugh at this guy, who is basically a Library of the Ancients enemy, because we just beat Omega and he thinks he can stop us. Laugh very hard. If you didn't beat Omega, you can still laugh.
A few Rapid Fires, possible Dualcasts of Bahamut, and this guy is helpless. His Magic Hammer is annoying, so you should deal with him quickly. Confuse, unlike Rocket Punch, does not go through Ribbon if you're still using a party of Freelancers and Mimes. If not, just equip Rapid Fire and spam it. However, he does cast Protect, so Bahamut is better.
Try and equip Learning on someone, or make someone Blue Mage. Because learning Magic Hammer from this guy isn't a bad idea, if fairly unnecessary.
Now, when you come out the Library, you are somewhere new. Interact with the book again to back to the cave whenever. If you're running low on items, or are not as prepared as you thought, this is a good spot to Teleport away and go back home. You won't have too much to deal with coming back all the way here, and with the bosses dead, it's even easier.
Whether you go back home or not, I'll meet you next page.