This page explains how to save Marbule and fight the Black Dragon in Chrono Cross: The Radical Dreamers Edition.
During your mission to obtain the relics belonging to the six dragons, you can undertake a sidequest to save Marbule from the spectral monsters that have infested it.
Otherwise, bring Fargo with you and head for the S.S. Zelbess in Home World. It’s still moored at the north-east side of the main island. Make for the captain’s cabin and speak to this world’s Fargo. Your Fargo (from Another World) will take over the ship and bring it next to Marbule.
From here, Nikki and his performers will hold a concert. It’s quite long, so sit back and enjoy the show. (Or fast-forward and mash the Action button.) When you regain control, depart the S.S. Zelbess. Fargo will obtain his Level 7 Tech, Invincible. (For the record, it doesn’t actually make him invincible.)
You’ll automatically arrive at Marbule. The once-spectral monsters plaguing the town are now very real–and fight-able. Without further ado, go around the place, mowing down all the monsters along the way. So long as you’re properly geared, you can probably enable Auto-Battle and fast-forward, while browsing the net or something.
After clearing away all the monsters that are outside, head for the cave towards the north-west and defeat the remaining monsters inside. If successful, a message box will appear, saying that a dragon’s cry can be heard. No dragon can be found here–not in this world anyway, so exit Marbule.
Next, head over to the portal in Opassa Beach to swap to Another World. Then travel back to Marbule (Another World). Return to the cave where the last bunch of monsters were–you should find the Black Dragon slowly waking up. Because of its long sleep, it’s feeling a bit rusty and wants to use you as exercise…
How to Defeat the Black Dragon
The Black Dragon is intimidating and has some menacing attacks, but honestly it could be far worse. For attacks, it can use Dark Breath and Gravity Bomb to hit all your characters and potentially inflict Curse, which causes stamina loss when hit. It can also use standard Black Elements, most notably Freefall.
Black Innates are good here, as they’ll take reduced damage from most of the Black Dragon’s attacks. This will naturally include Lynx. White Innates may sound good on paper, but keep in mind they’ll take more damage. Still, feel free to use them if you’re confident.
Regardless, White Elements are the way to go, but there’s a caveat. After being hit by a White Element, the Black Dragon will cast Anti-White. So make sure to equip an Angel Charm or pack Elements to cure it (Black Out and Panacea). This is important because it could prevent you using Recover All and Revive.
Towards the end, the Black Dragon can use Seal-All to prevent your party from using all Elements. Obviously, this will be awkward for healing. Thankfully, you can partially mitigate this by carrying Charm accessories; other than the Angel Charm, the Sea Charm and Forest Charm would be useful.
Also, you could place a Freefall trap near the beginning, to negate damage from Freefall. The Black Dragon doesn’t use it often, but any little helps (plus you get to keep it). Besides that, do your best to color the Field Effect White (or anything that’s not Black), while routinely topping up your party with Recover All, Heal All, Cure Plus etc.
After the Fight
For defeating the Black Dragon, you’ll earn the Grim Reaper Summon Element and the Black Relic.