First thing’s first: make sure you’re playing on Beginner Mode so Phantom Aqua has less HP. In order to reach each Phantom Aqua (herein referred to as PA to avoid confusion) you must complete a couple of puzzles throughout the mines, the never-ending staircase, and the infinite pillars. It doesn’t matter in which order you complete them, but be sure to save before beginning the third area.
Before getting into battle tactics, it is important to note that Kingdom Hearts 2.8 uses autosave functionality, and can also be soft reset at any time by pressing L1, R1, and the OPTIONS buttons all at the same time. This will immediately close what you’re doing and take you to that game’s main menu. This means if you fight PA and get hit, and need to retry (which let’s be honest: this WILL happen, likely many, many times), you don’t need to restart the system or finish the fight or lose intentionally in order to try again.
So if (when) you get hit, do a soft reset immediately and return to 0.2’s main menu. From there instead of loading up your save file and going through the third puzzle again, simply select Continue. This should load your game immediately before entering the mirror for the third PA fight. It took me literal dozens of attempts to beat her without getting hit, and a 20-30 second reload saved my sanity compared to 5-6 minutes of replaying a puzzle before the fight.
Moving onto the fight, you’ll want to move Aqua as little as possible. Let PA move around you. She will never attack from behind, so stay still and keep your eyes forward. PA starts the fight with two to four quick teleporting strikes, either from the left or right. A couple up close with the keyblade, followed by one or two long-distance blasts. With her keyblade attacks, wait until she’s halfway to you, then hit square to block. If the prompt comes up to counterattack, do so, but only hit her the once. Don’t try for a combo yet. When she readies the long-distance blast, there will appear to be three of her. When their keyblades are outstretched with energy at the tips, tap square and you should block in time. This all happens very fast, and will likely be the cause of many restarts.
Once you make it through the first series of attacks, PA will teleport in front of Aqua and walk slowly for a moment. This is the sign of her next move, where she creates several copies of herself all around Aqua, only one of which is actually her. As stated before, do NOT move. Do NOT attack with the keyblade. Every copy can be reached with Thundaga, so as soon as the copies appear fire off a blast. This will make all the copies dissipate, leaving the real target behind. Immediately after casting Thundaga, lock-on and attack. Don’t move toward PA; just attack. Aqua will automatically close the gap on her. You should be able to get two full combos off before PA disappears to begin her next assault. Quickly center your camera to face the middle of the arena again.
One of two attacks might happen next; occasionally PA will cast a series of Blizzaga blasts, but most often she’ll have more copies with her, spinning around Aqua as they use Spellweaver.
If you see Blizzaga, there will be three waves. If you time it right you can block all three blasts. No opportunity for counterattacks, but that’s okay. Just make sure to block as best you can.
If several PA’s start Spellweaving around you, wait patiently for a moment. When they start to slow down, tap that faithful square button. The counterattack prompt will come up, but don’t be overeager. Wait a fraction of a second before attacking, otherwise you’ll break out of your block too early and their attack will hit you. It might take a few tries to get used to the timing, but don’t worry. You’ll get there. Once you’ve countered you should be able to get another full-string combo against her.
That’s all of her attacks: teleporting strikes and blasts, doppelgangers, Blizzaga, and Spellweaver. Following the Spellweaver attack she usually either goes straight into another Spellweaver or another round of doppelgangers; either way, use the same tactics as outlined above, and the combos used at the end should be enough to finish her off.
Note that these attack patterns are based off of my personal attempts, and were the most common order of attack, but they are not gospel. PA may well shake things up and go out of order from what I have outlined, but if you stand your ground and know the signs that show what she’s doing next, it shouldn’t take you too long to get this. Just be patient, take advantage of the soft reset, and remember not to move around. Good luck!