Part 1- Essentials
Part 2- Item Synthesis
Part 3- Sephiroth and Data Organization members #13-#8
Part 5- Data Organization members #3-#1
Part 6- Duel with the Lingering Will
Data Organization Members #7-#4
Well this took entirely too long. This is going to be shorter so if there's additions someone wanted to see I could have the space to put them in. (The last part of this Is almost at the 20K character limit). That aside, let's get to business shall we?
#7. Saix: Saix is probably one of the easiest members to take down, as he has very few changes from his storyline fight. Cure and Reflect should be in your shortcuts, as usual (Your choice for the other two). You'll probably want 4x Full Bloom+ for this, as Saix is going to tear your HP to ribbons if he actually manages to hit you., and the attacks you actually need reflect for don't pop up that often.
To start the fight Saix will immediately begin charging his berserk mode. Start attacking him to disrupt this. Be careful about using a second finisher, as Saix may end up retaliating before you can react. When Saix recovers from stunlock, he'll almost immediately attack you. He'll rear back and utter the phrase "No Exit" before striking you, Either when he's reared back all the way or just after "No", block or reflect, which should hopefully give you another opening to attack, or at least keep you safe.
Saix will eventually manage to get into his berserk mode. As soon as the bar is full, get away from him before he damages you with his battle aura. Saix then begins his rampage around the arena, slamming his weapon about. Keep your distance by gliding around the arena the whole time while he's in berserk mode. Saix primarily uses two attacks during this phase. The first has him lunging about screaming "Hnngh!,Hungh! Heaargh! BEEEEGONE!".After the third grunt, prepare to reflect as Saix is about to throw his claymore down at you . If you timed your defense right, you should be able to grab his claymore with a Reaction command. The second attack has him run around slamming his weapon into the ground repeatedly to make shockwaves. When he's made his final shockwave, Saix screams "MOVE ASIDE" and proceeds to throw his weapon at you. Reflect or dodge and use the reaction command to grab his sword...club...thing. If you successfully performed the reaction command in either case, immediately go up to Saix and start pummeling him with the reaction command. You have only a couple of seconds at most before the weapon disappears from your hand this time, so don't dally. After the reaction command is done, Saix will say 'I misjudged you." and counterattack. Guard or reflect, which usually lets you grab yet another weapon to use the reaction command again. He may also use his "No exit" counterattack as well, so be prepared.
When Saix has been knocked out of his berserk state, he'll revert to his behaviour from the start of the fight. Rinse and repeat the strategy until you whittle his HP down to the halfway mark. Brace yourself for his second, more dangerous phase., marked when Saix screams "ALL SHALL BE LOST TO YOU" as he enters berserk mode.
This time Saix is significantly more dangerous, as his range has increased considerably. Your strategies are laregely the same as before, however the length of his attacks is much longer, so you'll have to avoid his attacks for much longer before getting the opening. Since he has a wider range, dodging is significantly more difficult than before as you have to stay further away from him at all times. To that end you need to keep track of where Saix is at all times. If you find yourself far enough away from Saix to safely land, briefly do so and jump back up to regain height for your glide, as generally speaking the higher in the air you are the easier it is to avoid his attacks. Saix's lunge attacks are likely going to be your main source of problems, as they now send embers of blue fire flying about. There sadly is no real way to counter this other than keeping far away from him at all times.
Saix isn't outright "easy" (None of the data bosses save for maybe Lexaeus and Marluxia really are, of course), but he's fairly predictable due to his very limited move arsenal. Saix drops defense boosts, so if you find yourself struggling to stay alive against the other data bosses it might be worthwhile to grind him for them.
#6. Zexion: Zexion is probably the single most annoying boss in the game, as he uses a bevy of cheap tricks and interface screws in order to kill you. Accessory choice is mostly irrelevant, as your primary source of MP regeneration is going to be items this time. Equip a large amount of MP restoring items (Ethers, elixirs, Megalixirs) on everyone, and see to it that your healing items are equipped on shortcuts alongside Curaga and Reflega. Make sure both Donald and Goofy have limits available for use. You're going to need them.
Zexion opens the fight by sealing your teammates into books. As tempting as it is to try and free them, pay them no mind, as no matter what happens your party is absolutely going to get sucked into the books. Instead, start to whale on him to get a good bit of damage on him before he begins the much nastier part of his fight by sealing you into a book yourself.
As soon as you're sucked inside the book use reflect or start a limit (you can prepare to fire off the limit before you're sucked in so all you have to do is press the confirmation button when you're in there), as his first move is usually to trap you inside yet another book, which drains your HP incredibly quickly and is all but guaranteed to kill you even with Second Chance+One More. Alternatively he may end up turning your command menu into a trap, forcing you to scroll through the pages to find the commands that will heal you and set you free. (Healing is usually marked by synonyms of the word "restore" or "renew", and the commands that free you are mostly synonyms of "dispel" and "release". Whatever happens, when you finally get a chance to act, begin attacking the books to try and find the real Zexion. The real Zexy often reveals himself when attacking, his book glowing red when he's casting a spell. If you spot this glow, it may be worthwhile to reflect immediately to repel the oncoming attack.
I admittedly also have a lot of trouble dealing with the attacks inside of the book, so these strategies are going to be very far from perfect. Anyway, Zexion will use one of several attacks to assault you. The first of these is launching books at you from various directions. These will have to be reflected or avoided using a limit. If you get hit, wait until you touch the ground to recover, as the books won't be able to kill you if you have the SC+OM abilities equipped. This also goes for his shockwave attack, which is avoided the same way. Zexion also gas a vortex attack, turning the books into a cyclone of pain which the Zexion book orbits around. Find him and pummel him.
When you finally deplete the illusion book's HP bar, you should get a reaction command. Use it to return to the real world. This time he can suck you in before your party members, so when the green emblem is up make sure to stay well out of his line of sight and make sure you aren't in front of your party members.
Zexion's ultimate attack is a variant of Sephiroth's Meteor, which is used when inside the book and Zexion is at 50% or less HP. Before he actually hits you with it, you have to play a game where Zexion will show three spotlights. The book he uses will teleport between the three spotlights, the one it's in being blue and the other two being red. The book will shuffle positions 13 times (credit to Kinsei for bringing this to my attention). When switch 13 happens, you'll have to very quickly enter the spotlight. If you guessed right, you should be safe from the meteor storm and have an opportunity to deplete the Book's HP bar and hopefully escape. In the case that you miss, there is a way to survive the meteor storm. If you prepare to activate your limit while the book is shuffling, you can hit the limit command as soon as you fail and make yourself invulnerable to the entire attack. If you have Second Chance+One More, you can also just let the storm pummel you and use cure afterward.
Zexion is very,very,very annoying and is probably one of the more troublesome Data battles to master. Thankfully the borderline-broken properties of limits make this fight much easier to survive, so if you can get a good handle on his most dangerous attacks, this shouldn't be completely hair-pulling.
#5. Lexaeus: In rather refreshing contrast to Zexion, Lexaeus is much more straightforward, and generally comes up as one of the two candidates for "easiest data battle". Accessory choice doesn't really matter, though you may want to choose Shadow Archive+ for the boost in Reflect damage. Curaga and Reflega are your time-tested standards for shortcuts. You might want to stock some ethers though to ensure you can keep reflects going the entire fight.
Lexaeus begins the fight by summoning his battle Aura and beginning his first Axe swing. For this attack, Lexaeus will scrape the ground with his weapon and smash you with it. Despite Lexaeus's strength, this attack is easily blocked. You should get a reaction command to dispel his Aura. Use it and slam Lexaeus with a combo. I'd advise not using a second finisher, as Lexaeus is likely to retaliate should you do so.Lexaeus will then lunge at you with his weapon. reflect From this point forward Lexaeus will use one of these two attacks alongside a third one, that being his quake attack, where he flings his weapon high into the air, catches it, and causes chunks of rock to fly up. Watch the axe and as soon as Lex catches it reflect or dodge roll. He does this in a fairly rhythmic pattern so it can be easily dodged with some practice. It's advised you reflect the final surge to do some damage to him, as he briefly becomes vulnerable to damage at the end of the attack.
After enough Damage Lexaeus will strengthen his aura (Hitting 32,000 on his power meter). This is when he actually starts being a threat. When Lexaeus finishes charging his power, he'll run right for you, glide far, far away from him in order to keep him from grabbing you (which for a low level player is guaranteed death). When he finally stands still make sure to get near him (but not too close so you can avoid his grab) again so you can counter his attacks properly. His attacks are mostly the same during this point, only being much more deadly and having far superior range, so attempting to run away to avoid damage is useless for the most part.
Lexaeus's final move is a rather laughable meteor strike. Lexaeus flies outside of the arena, and then flies towards the center of the arena. Use reflect four or five times, starting right before he lands to avoid it and be prepare for a followup attack (usually his quake move)
Lexaeus is extremely predictable, barring his grab attack, so this fight isn't especially difficult as long as you have your wits and composure about you. This is probably the best fight for a level 1 player to grind Lost Illusions.
#4. Vexen: This guy is simultaneously very tricky and deceptively simple. If you tackle him the old-fashioned way, you'll quickly find that Vexen is very, very difficult. Vexen's shield can be very difficult to break due to it being immune to magic finishers. Vexen's data reticule is also stalking you the entire time, so you often have very few chances to attack without risking data collection. This often means that you'll spend long stretches of the fight trying to separate Vexen and the Data collector. In addition to this Vexen has a powerful array of defensive attacks, including long-range anti-air icicles and his buzzsaw snowflakes that he often uses to try and stunlock you, and in most cases as soon as Data-Sora is created you can kiss yourself goodbye, as it's extremely dangerous even if you're able to repel it with fire attacks. I've only ever beaten this data battle once without being at 99 because of how demanding and luck-based it can be normally, so I'm going to tell you specifically about the "Kinda cheating but still takes skill and is necessary at level 1 no matter what" strategy, as demonstrated by Bizkit
here.(And of course check out the rest of his stuff, as he's one of the best players around and covers some quirks and additional strategies for bosses, with the added benefit of you actually being able to see them done firsthand.)
Pack a good stock of Ethers or Elixirs, as you'll absolutely need them. Make sure you have sliding dash and one (and only one) combo plus equipped in your abilities. Reflega, Curaga, Firaga and your ethers/elixirs should be set to shortcuts. Wear all your Shadow Archive+, so as to give yourself additional magic damage. Donald should have the Comet Limit equipped
At the start of the battle you'll want to lock on and immediately attack Vexen using Sliding dash. As soon as it lands use reflect to block the snowflake attack. Vexen will then levitate away from you. Follow him with another sliding dash and use a finisher to break his shield, if all went well. If you managed to break his shield, use fire right away to send him into the air. If you can, go into Final Form (make sure to pause buffer so it loads before Vexen recovers!). Regardless of whether or not you transformed, wait for Vexen to hit the ground. As soon as he does, use fire again. Vexen is completely stuck in a cycle of pain as long as you make sure he touches the ground before you hit him with Fire again. When you're about to run out of MP, very quickly use your MP restoring item right after you use fire. Doing it fast enough allows you to continue using the attack uninterrupted. When Vexen is down to 5.5 bars of HP, get the hell away from him, get out of Drive and immediately use Donald's Comet. If this was done properly, you and Donald should enter your limit, rendering yourself invulnerable to Vexen's desperation move. Use an Elixir (or ethers) to restore your HP and go after Vexen. Get his shield's HP down as much as you can before the limit is about to run out. Back away from Vexen and prepare to use your limit again immediately, (Failure to do so is likely to end in you being frozen and killed) and of course refill your MP right after you start the limit. When Vexen finally chills out and ends the move, prepare to move in to break Vexen's shield before he can summon Data Sora (who's at full power and not a customer you want to deal with.). Use Fire to stagger him and enter Final Form. If you were successful, Vexen is basically dead at this point, so long as you remember to let him touch down between each Firaga. This method is tricky, but hopefully shouldn't give you very much trouble at all.
If you're going to do this the normal way, remember that you absolutely need to be patient to stand a chance. Sliding dash, Slapshot, guard break and finishers (it's okay to have multiples in this fight) that send Vexen flying are going to be ideal. The former three help with breaking his shield while the latter buys you time to either start Firaga or get in a Keyblade combo. You are, of course, free to milk the Firaga spamming method for what it's worth so long as Data Sora isn't active. Keep in mind that the Ice spires can be avoided with Dodge roll, while the Snowflakes can be reflected. Those are his two main attacks. Vexen's DM can either be tanked (somewhat unreliably) with Second Chance+Once More or you can make use of Limits just like the "cheesing" method.