These are my addendums to that guide, these contain easier strategies or corrections to the guide:
Back Row+ Sadness are your friends. Learn to love them. Use them in EVERY boss battle unless otherwise stated.
Air Buster is a lot easier if you just use Tifa as a kamikaze (have her limit break bar full before the battle, then after she uses it she will get her limit bar filled again by the counter attack, then after the second counter she's dead. Then carefully attack his BACK. Whoever is facing its front should just use Defend. Once he can't turn anymore he's toast. You may revive Tifa at this point.
Cover Materia is essential.
Locke/Venus/Theo at the Shinra Building: First, climb up the floors, get the Hardedge from the SOLDIER:3rds, then come back down. Before starting the battle, have Fury on Barret, Sadness+Cover on Tifa, and Steal on Cloud. Fill up your limit breaks and start the battle. Tifa should Defend, Cloud should Cross-Slash Theo, Barret should Mindblow Locke, then Theo, then Venus. After he's done with Mindblow use two Tranquilizers on him. Meanwhile Steal their stuff, get Theo first as he's more than likely the first to die. If you're panicking, just chuck some Loco Weeds at them.
And yes, definitely get the Elemental Materia.
Summons are pretty much worthless to me. Silver Chocobo used that odd attack that has no effect a bunch of time, I think this is probably a bug.
Get Long Range Materia, it is essential.
Get L4 Suicide while you're around here. No way you can get Beta right now. Matra Magic is also useful for the beginning of the game. Steal Ethers from the Kalm Wolves.
Get the HP Plus from the sendoff.
Use Cloud/Red/Aeris for Jenova-Birth. Equip Aeris with the HP Plus and a Headband. Spam Healing Wind over and over again. Cloud should be your main attacker and Red should focus on supporting Cloud/Aeris, using Lunatic High when it's available.
Beta can wait until the Tiny Bronco... nevermind the Zolom.
Laser, Frog Song, White Wind and to a lesser extent Mighty Guard are essential, along with 2 HP Pluses at Cosmo Canyon.
You don't need to wait until Rocket Town to beat Lost Number. Just outside Nibelheim steal 3 Gold Armlets from the Askendimés (a new enemy!).
Get the ???? (question marks) Enemy Skill as it will be one of your best attack tools for most of the game. If a character needs a heal, wait until it's their turn, cast ????, then heal the character. Works wonders.
Then go tackle Lost Number, poison him ASAP. Sometimes you will have to hold on with dealing direct damage in this battle and just wall up and heal.
Get the Counter Attack materia and use it, it's essential.
Go learn Beta at the shores of Bone Village/Temple of the Ancients. It's a new encounter with a Tonadu and three Yptilos (crab-like enemies). Equip fire resistance, frog song everyone except the Yptilos you want to manipulate so it can cast Beta on you. Kill the Tonadu with the Yptilos' other fire-based attack (will deal about 7k to it!) then just pick them off. Much easier than the Zolom.
Get a few items like Right Arms and T/S Bombs then go do the sidequest at Wutai. Aeris' Limit Breaks are NOT essential. Infact, I pretty much stopped using her at this point.
Learn Magic Hammer, which is also essential. Once you're done you have access to the best AP spot for a good while. With double-growth weapons and armor fight the Adamantoises. Manipulate them and Steal (not Mug) the Adaman Bangles, then just make it kill itself with Light Shell (will take a while...). Then you'll get a whooping 1600 AP. You will come back here later on for some actual Materia leveling, so just focus on the Adaman Bangles AND the Death Force Enemy Skill, which is also essential.
Learn Aqualung. It helps.
After you're done with the Temple of the Ancients, go tackle the Kujata boss. Get some Fire Veils at the Wutai item shop, and Spider Webs from the Kimara Bugs in Gongaga. Equip one character with a Peace Ring: he/she will be your main attacker. Get some resistance to Ice and Lightining (Fire is not needed as you will see why). Now as soon as the battle starts, cast Barrier+All, then MBarrier+All, then start depleting the Hadadigor (the Ice horse)'s MP with Magic Hammer. If it casts Haste immediately toss a Spider Web their way.
If they cast Reflect on you start healing with White Wind. After Hadadigor's MP is done then start draining Yamigor (the Lighting horse)'s MP. Then after they're both depleted of their MP toss Fire Veils to kill of Hadadigor. Now with one of them gone, kill Levant with ???? with the person that has the Peace Ring equipped. It will only counterattack that character and will not use Levant Wind on everyone, it will just use Charge over and over which won't be as annoying with only one of the horses alive. It has little HP which won't be a problem for the defense-ignoring ????. After Levant's dead the last horse will be a sitting duck. (no pun intended).
Jenova-Life is annoying. I personally killed it by draining all of it's MP. Adaman Bangles are necessary. You might want to get the third Enemy Skill materia first and load it up with useful skills before you try this.
Snow Queen is stupid. Load up on resistance to Ice-elemental attacks (I did not have the Ice Ring yet but I had two 2-star Elemental materias). I had Cloud, Vincent, Cid as my party (can be done with any party, but just to illustrate...). Cloud got killed, Vincent and Cid were confused by Envy... and besides Vincent and Cid hitting each other for a measly 100 hp out of 1500 hp, she couldn't do anything to them. She doesn't have much MP so she pretty much wasted it on them, while they could just keep healing with Potions and Hi-Potions. After she went out of HP, I revived Cloud and killed her for the win.
Learn Bad Breath, Magic Breath, and Dragon Force (either do it legit, by reflecting it, or hack it in since Gjoerulv's said he thought he made the Blue Dragon manipulable.) If you have trouble with the Icicles just use Bad Breath+L4 Suicide when it's Icicle+Bats, and then Laser the Icicle since it's weak to Gravity (Laser + one single point of damage will kill it). If it's Jokers, Bats and Icicle just Laser the Icicle ASAP.
A trick with Magic Breath: as you know, if an attack has multiple elements, if a target absorbs even just one of the elements, it will absorb the attack. So if you absorb one of Fire/Ice/Lightning, Magic Breath will HEAL your MP. It pretty much refilled my whole MP :p
Kill 2 Blue Dragons, get 2 Dragon Armlets, then trek back all the way to the Tiny Bronco. Schizo is definitely the turning point here, I couldn't just scrap it through by now as I did. So here's what you do: decide on a fixed party for the rest of the game (as usual, Cloud/Barret then choose between Yuffie or Cid), buy 3 HP Plus materias for each character, then wear Double Growth armor and weapons and kill Adamantoises for AP (800exp/800AP/1600AP with double growth, perfect so you don't overlevel), get a few levels and AP (but don't overdo it) with the Sea Worms at the Mideel beaches (cast Bad Breath and L4 Suicide and it's dead). They net 5000exp/500AP/1000AP with Double Growth. I killed so many Adamantoises I actually mastered one set of HP Plus materias.
By this time it's the best time to get Omnislash as the Battle Arena enemies will be the Disc 1 enemies. Give Cloud a Dragon Armlet, Enemy Skill (with the most skills learned, Dragon Force, Death Force, Mighty Guard, Bad Breath, Magic Hammer, White Wind, ????, Frog Song are essential), Counter Attack, the higher leveled HP Plus materias, Restore materia and Destruct materia for DeSpell, plus Mug+Added Cut (ghetto Double Cut). If you manipulate the slots (
http://www.youtube.com/watch?v=uGNYMHdE4ks) it will be easy to get 10k BP a pop. Also, be sure to buff up with Dragon Force and Death Force on the first battle of the arena, plus wear a Peace Ring, and have plenty of restorative items around (for statuses, Remedies stolen from the Velcher Tasks near Rocket Town are a must)
I also got Angermax for Barret and Doom of the Living for Yuffie. I did the Pagoda by now, probably a bit overlevelled but oh well. Barret should be your tank: Cover+Counter combo is essential, and Yuffie should be your mage, Sprint Shoes work really well because of her naturally high Dexterity.
Take on Schizo, gear up on elemental resists. He'll still be annoying but not as much as before. GET THE PROTECT RING. You'll need it later on.
Gold Dragon: blast it with Laser then kill it ASAP. This battle gets only harder the longer it goes on. Save yourself the trouble.
As for Jenova-Death, gear up on Fire resistance, cast Death Force the first turns, also Added Effect+ Mystify/Peace Ring will be useful. Tropic Wind will heal your MP if you absord Fire! It actually ran out of MP... and as with Jenova-Life, once it does it's harmless.
For the Corel Huge Materia train, you need to be really prepared. I had Cid/Barret/Yuffie. Barret with a Protect Ring. Up battle speed a bit, then just max out battle message speed. Now be prepared for a lot of resets. Get rid of the first set of two enemies ASAP, then get rid of the Wolfmeister with Bad Breath and ????+Aqualung. Then for the Eagle Gun get rid of it with Stop, plus ???? and Thundaga.
Now for Locke/Venus/Theo, they have a long intro so max battle message speed with save you loads of time. Then Bad Breath them, wall up and STEAL THE ESCORT GUARD FROM LOCKE ASAP. There are only 2 in the game! (and one without this mod!) Theo has a Mystile too, but you can get 3 of em anyway. Once you've stole the Escort Guard, finish them off with Limit Breaks and ????.
Phoenix boss, just manipulate one of the birds, have it cast Angel Whisper on your 3 characters so they can learn the Enemy Skill. Now, have someone with a Mastered Cover materia, drain that character's MP, then start draining their MP. Once they're out of MP they will only use an attack that drains MP... but it can be Covered. They cannot drain MP from a character that has 0 MP, so they'll be powerless. If they do have enough MP for an Angel Whisper when they die, they will start to revive eachother, so beware.
Quadra Magic boss: by this time you'll have Cloud and the Highwind, so do the Highwind trick to land on the Quadra Magic peninsula, then go fight it. Equip the Escort Guard on Barret, plus Elemental resists for Fire and Ice. You'll notice that Adamantai's Magic Chain cannot hit Barret: Freeze will heal him, Flare will heal him, Tornado will miss, and Break will miss. Now blast it with Laser: it will start countering with Full Cure when it's at low HP, so just keep using Laser then finish it off with some magic attack.
I'm tired for now, if you people would like any other strategies from me for the rest of the bosses just say so.