Aqua meets Scrooge McDuck upon her arrival at Radiant Garden, who gives her a lead on Terra's whereabouts.
Search for Terra and pursuit of Unversed
Open a chest to the right for a Potion and one to the left for an Ether, then take the exit north of here to the Gardens. To the left of this area is Zero Gravity, and up the steps are two more treasures: Strike Raid right by the staircase to the left and a Wellspring Crystal down by some trees to the right. Past the next set of stairs is a Fleeting Crystal to the left. Walk through the gate to the next area, the Front Doors, where you should find a Map, a Panacea, and a save point. Head up the stairs for a scene in which Aqua meets Mickey and Kairi. Your new objective: protect Kairi from the Unversed with Mickey on your side. This shouldn't be too difficult, especially with the frenetically-attacking new party member to aid you. A scene follows the battle after which Aqua forges a D-Link with Mickey and obtains the Destiny's Embrace Keyblade. Head back up the stairs to retrieve Mine Square from a chest, then exit back to the Gardens for a short scene.
Head down the first set of steps and rather than backtracking entirely, follow the staircase to the right to a busted door. Through here is a boss battle.
Boss: Trinity Armor
This is a boss battle fought in both Terra's and Ven's stories, exactly the same aside from the few advantages Aqua possesses in battle (her ability to guard from all directions should make this battle much easier). To reiterate, Trinity Armor is made up of multiple parts, each with their own health gauges and unique attacks. Focus on attacking the arms or legs first, as the body has the most health and will take the most time to get through, and taking out its limbs will limit the variety in its attacks. With its arms, it guards its body and charges at Aqua, an attack that she can Cartwheel away from. With its body, it can create a massive laser that it sends around the arena by turning; Cartwheel into this to dodge through it, or Cartwheel away if you feel you can create a comfortable distance in time.
When the Armor splits, it is at its most vulnerable, but also its most dangerous. Attack a body part far away from the others, but watch for any cues. If the head takes center stage and starts spinning, it will pelt out bullets in all directions. Guard regularly, as this attack can be fatal if Aqua gets caught in it. If all parts start spinning, they will slide across the arena aimlessly and damage anything in their path. This is another attack that can prove fatal if not avoided properly, the safest approach again being to guard constantly. When only two parts remain, the Trinity Armor will form a laser between them and attempt to trap Aqua within it. This can also be dodged by executing a Cartwheel into it, but you should instead focus on attacking here by hitting the parts' exposed backs. When only the body remains, it will spew balls of magma in all directions. This can also be dodged with a Cartwheel, but at this point, you should focus on finishing it off. Do so for the Bladecharge Command Style and a scene, as well as a lifetime pass to Disney Town.
Aqua is left alone by an angered Terra and Ven following the battle, so with nowhere else to go, take the exit to the Aqueduct. A chest to the right between some bushes contains a Hi-Potion. Down at the very end of the path by the gate is a Block Recipe and Thundara. Continue down the bridge to a Hi-Potion and an exit to the Fountain Court. Follow the platforms right until you find a working fountain furthest right; this should launch Aqua up to a ledge that allows her to reach a platform in the corner. Here she can open two chests for Counter Blast and a Soothing Crystal. Drop down to ground level and take an exit up some stairs to meet Merlin. Enter his house and Examine the book for a new Command Board, and open the chest by the entrance for Xehanort's Report 3. Head back outside and take the exit at the other end of this area to Central Square, where Aqua will encounter a malicious individual who Ventus claims to have stopped.
Vanitas' attacks are pretty much the same as they were in his fight against Ventus at the Badlands and his fight against Terra with Xehanort at the Keyblade Graveyard. He will sometimes fade away after being attacked, leaving an afterimage before dropping down from behind Aqua with his Keyblade. Perform a preliminary Cartwheel before completing a combo in order to avoid this whenever possible. From afar, Vanitas drops dark lightning from a jump, spreading it out toward Aqua in an attack easily avoided with a Cartwheel. He also tends to shoot a fireball that splits into three, homing in on Aqua; this is also easily avoided with a Cartwheel, in particular one right through the fireball, giving Aqua an opportunity to surprise Vanitas on the other side of the fireball with an attack of her own. Aside from these, Vanitas will perform the usual quick physical combos with his Keyblade. These can be blocked with Aqua's Barrier and followed up with a Reprisal Command, such as the recently-acquired Counter Blast.
As far as Magic goes, the usual projectile spells such as Fire and Blizzard should be helpful alongside physical Commands that can be initiated from afar, such as Fire Dash or Ice Barrage. Vanitas is particularly weak against any variation of Mine attacks as long as Aqua makes sure to lead him into one (which shouldn't be too difficult considering his tendency to enthusiastically pursue his targets); these do a good job of stunning him into the air, leaving him open to more attacks. He is also weak to Magnet/Aero/Zero Gravity, floating into the air without any control once caught, and Sleep, dozing off for a bit when Aqua successfully casts it on him with complete vulnerability. As far as future battles go, make sure to get a grasp of Vanitas' attack patterns and construct a workable strategy, as this is only Aqua's first battle against the masked boy.