This isn't a full guide, but I wanted to detail my setup that made me invincible around level 15 of the dungeon.
Before you do anything... pick Yuna! You'll thank me later.
I used four accessories:
1. Confuseproof: You can make this almost immediately. You get the ability by wearing a Gunner and a Dark Knight. Trash the Gunner dress sphere as soon as the accessory drops. You'll want Confuseproof for the final boss, but it's nice to have earlier and easy to get.
2. Stealth Cloak: make this by wearing Songstress/Thief/Psychic. Do this ASAP - it lets you avoid all traps in the dungeons and makes the game that much easier. No traps means you really don't need to keep any sort of status curing or curse curing items, and it frees up your inventory that much more. If you really want to be safe, carry an Alchemist dress sphere just in case to cure any curses, but I never had to use mine once. After you get the accessory dump the Songstress and Psychic, but hold on to Thief for level 44.
3. Doomproof: honestly, you really only need this for the Tonberry room, but it's nice to have since Doom's really the only thing that will ever kill you. Make it by wearing Dark Knight/Trainer/Samurai. After you get it dump Samurai.
4: Triple Tiara: steal it from The Founder on level 44. Wait on the elevator for him to appear (it takes forever, like 400+ moves), then steal it from him before he turns you into a Syndicate member. As soon as you have it, use the elevator. Lets you attack three times every round.
I always wore the Stealth Cloak, and adjusted the other three accordingly based on situation.
Now for dress spheres. My main setup was Dark Knight, Mascot, Gun Mage, White Mage, Black Mage. This setup is awesome. Make Dark Knight your main sphere, and the other 4 as support. Level only the Dark Knight sphere. Some highlights of this setup:
* Dark Knight has tons of HP, tons of defense, and two overpowered moves: Black Sky and Doom. Both take HP to cast... and you can cast Curaga for 5 MP to get them all back. Doom kills almost anything, but you don't get EXP if they die with it. I used Black Sky to kill everything in a room until it stopped one-hitting everything, then I switched to Doom in the later levels.
* Curaga! You won't need any silly healing books.
* Recover MP while you walk, so no need to carry Ethers and Elixirs.
* Take only half damage. The only thing that will hurt is magic, but even that doesn't do too much. Plus, you'll have so many HP you'll rarely need to heal your dress sphere, if ever.
* A Demi move that you'll love having against the final boss.
* A move that lets you push enemies away that get too close.
Two other important points: play as Yuna, and get the Trainer dress sphere ASAP. Yuna has a special ability with Trainer that Rikku and Paine don't get: Dig. With Dig you can find up to 5 items per map for 4 MP a try. You can find anything, even overpowered books and items you shouldn't have yet. I was able to get my Dark Knight dress sphere up to level 99 by floor 25 because I kept finding books that let copy my dress sphere and double it up every time. Yuna's level also rose pretty quickly, because I kept finding potions that would raise my level.
Last thing: use a guide like
http://www.gamefaqs.com/ps2/918914-final-fantasy-x-2-interna... for the elevator requirements. Every 5 levels you have to meet certain requirements to use the elevator. They're easy, but if you don't know what they are ahead of time they're a huge headache. Also, that guide does a decent job of outlining the basics of the game. Read it for that, then use my setup and you'll breeze through. And it goes without saying that you should take the free save every 5 floors in case something bad happens like a destroyed accessory or worse.
Good items to have on hand:
* The Hope book that resurrects you when you die. Try to carry one at all times. Two when you fight the Tonberries.
* Make sure you have two Save books for the Tonberry treasure on floor 61.
* Make sure you have three Secret Room books for the Tonberry treasure on floor 61. I didn't find these until somewhere in the 50s, and I had to duplicate some with Notebooks to get enough.
* Dress Fusion books, until you max out at level 99 for Dark Knight. After that they're useless.
* Dressmaker books, until you have all the accessories you need and the final dress sphere setup you want. After that they're useless.
* Keep a Thief dress sphere to steal the Tiara on level 44 from the Founder. After you do this you can trash the Thief sphere.
Let me know if you have questions. I don't know if anyone will actually read this, so I'm reluctant to write more!