In order to trigger this battle again once you have completed the game, you must play through again from the beginning. Like with any boss, it is gone from the world once you defeat it. If you play on Beginner Mode (which I strongly suggest), it should only take around half an hour or 45 minutes to rush through the Castle of Dreams and The World Within to reach this point.
Important to note is that you don’t just have to beat the Darkside that supports the Dark Orb, but also make your way to it through the flurry of aerial blasts on the path leading up to it. If you touch any of the blasts as they fall to the ground, you’re hurt. If you touch the energy balls when they sink into the ground and expand with spikes, you’re hurt. If you touch the lingering swirly, red and black pools they leave in their wake, you’re in the clear. Try to wait to walk over them until the spikes have disappeared, then run right over them. It’s a good idea to stay more on the ground than in the air for this bit, as you can see the attacks coming pretty easily from the ground and dodge accordingly.
As the video above shows, the key to winning this fight damage-free is keeping your distance and using long-range attacks. As soon as you’ve reached Darkside hit it with a full-strength Shotlock to quickly take down a couple full HP bars. Even if you mess up mid-way through like I did, you’ll still have a great start to the fight.
Once you’ve got that first hit off, Darkside should be on its knees, attacking Aqua with its standard purple energy orbs that fire from its chest. Make sure you’re locked onto it and block the orbs to send them back at it. Usually it only fires off two or three orbs at a time, but sometimes you’ll have five or six coming at you, which makes for another nice chunk of damage to the boss.
As Darkside stands up from its orb attacks, use all of Aqua’s MP on a full magic assault. Firing off a barrage of Firaga and Blizzaga should knock it down almost another full HP bar. There are other minor attacks Darkside uses for the first half of the battle, such as assaulting Aqua with more glowing red orbs like you had to dodge on your way to fight it, but if you’ve followed this guide properly so far, its health should be low enough to completely bypass that segment and move on to it’s next attack.
Darkside’s upcoming laser attack is signaled by a small earthquake, which deals no damage – don’t worry about jumping to dodge it. Just keep an eye out for the six lasers that will soon strike out from Darkside. They don’t move in a straight line, so stay on your toes and dodge as necessary. The lasers come out from Darkside twice, then once more from the outer edges of the battle area.
Following the lasers it will go back on its knees to fire more chest blasts. Block them like last time while locked on to send them right back at it. By now your MP will have completely replenished, meaning it’s time for round two of your magic assault. Please note that in the video above, the casting of Curaga around this point was a mistake, and as you should have taken no damage up to this point, is completely unnecessary and a waste of valuable MP. Try not to do that like I did.
The pattern is pretty standard from here until the end of the fight: dodge lasers, block purple chest orbs, assault it with magic, repeat. If your opening Shotlock was a successful three Good or Excellent hits, Darkside should be defeated now. If you messed up the Shotlock, one more round of lasers, blasts, and magic should do it. If you see an opening and Darkside’s HP is low enough, feel free to finish the fight with a few strikes of the Keyblade like I did, but if you’re happier keeping your distance, that’s probably a better idea. No sense in getting cocky right at the end and getting hit unexpectedly.
Remember that if you accidentally get hit at any time during the fight, you do not need to reload your save file and fight your way back to this battle. Simply perform a soft reset (R1+L1+OPTIONS) then select Continue from the main menu, and you’ll find yourself right in front of the ivy branches just before that battle’s cutscene.