Galbadia Garden and the Sorceress are waiting in the forest just outside of Edea's House. Make sure you are prepared before flying into it: you won't be able to return to the world map for a bit.
As the battle is set to begin, Squall will be prompted to issue orders to his Garden. You can't make any wrong choices here, but if want to be the best leader possible, I suggest the "Prepare for the attack, Prepare our defense, and Take care of the junior classmen" options. Take the elevator down to regroup with your party members and form a party between Quistis, Selphie, and Irvine. While not particularly important, whomever you leave out will then form a party with Zell and Rinoa.
On 2F, talk to all the Garden members around to assign them to their areas and try to catch the junior classman before he enters the classroom and he will hand over a Cottage item to Squall. If you miss him, just exit and then reenter the screen and he will be walking back to the classroom again.
Head for the quad area of the Garden to find Zell rallying the troops there. Zell will request Squall hand over his ring; which Squall begrudgingly agrees to. Rinoa will enter the scene and then Squall and his party will return to the bridge.
The battle between the two Gardens will then begin...
With Seifer leading the Galbadian Army for the Sorceress, we can kill two birds with one stone, huh?
Galbadia has motorcycles, flying machines, and, apparently, kamikaze troops, and we've got swords and junior classmen. Perhaps Balamb should have prepared a little bit more for this. Anyways, you will be in control of Zell's party and there are now random encounters inside the Garden.
A new enemy, the Paratrooper, will be here, but he is only an annoyance with his status spells. Other than the Paratrooper, inside Balamb, and then soon, Galbadia Garden, marks the next-to-last appearance in the game for the SAM08G and GIM52A enemies. To continue the story, take Zell off-screen to the left of the quad and the colliding Gardens will cause Rinoa to fall and cling to a cliff below. Make your way to the directory and follow Squall's party to the front gate where you will assume control of them.
In control of Squall's party now, hurry to the 2F classroom to protect the junior classmen. You'll fight a battle with four Paratroopers and then be summoned back to the bridge by Dr. Kadowaki.
It will be decided that the party must make an assault on the Sorceress inside their Garden or the battle will be lost. The Doctor will encourage Squall to give a speech to raise the moral of his troops as you prepare to infiltrate the Galbadian Garden. Zell and the rest of the party will find a way inside, leaving Squall to rescue Rinoa.
Back on 2F again, Squall will be asked to look for a junior classman that has run off. You'll find him just outside the exit out to the deck, but a Paratrooper in one of the flying machines will attack Squall. When prompted, have Squall select to "Look for another option" and then "Press the button for the emergency exit" to engage in a mini-game-like battle. This thing is annoying due to the shifting camera, but thankfully, you can lose and retry with be no penalty (And no reward for doing it in one try). Try to land two quick punches, then just block. Once the option for the Deathblow appears, wait for the Soldier to block and then use the Deathblock to finish him off. After the battle, Squall will use the machine to rescue Rinoa and they will cross the battlefield to the, unguarded (?), entrance of Galbadia Garden.
Rinoa will reveal to Squall that Zell gave her his ring and she will ask if the lion on it has a name. The name-entry screen is puzzling to some players as it appears to be of absolutely no use. However, the name that is given here will be the name of one of the final bosses of the game.
Before proceeding, outside there is a hidden Aura draw point in the upper-right of the screen. Inside now, the party will be reunited and you are free to explore the Galbadia Garden and find Seifer and the Sorceress. As for enemies inside the Garden, there's nothing special; save for one, and the two previously-mentioned machine enemies. Also, I repeat, after this dungeon, Rinoa will be lost to you for a fair amount of time, so I recommend that you build her levels up much higher than the rest of the party.
From the save point, take a right, then another right and up the stairs to find Raijin and Fujin. They don't want to fight this time, but ask that you take care of Seifer.
From Raijin and Fujin, take a left and then into the dorm-room on the right. Inside, talk with the student who will give you a Card Key [1], although this is not an actual item in your inventory.
Return to the save point, then take the path to the left, followed by unlocking the left-hand-side door with the card key. You'll then be out on the Galbadian ice hockey arena. Here is the only place in the game you can encounter the Slapper enemy, although there is nothing unique about them other than their rare appearance. How come Biggs and Wedge got a card but not the Slapper? I digress...If you'll recall, in the locker room on the left-side, there is a useful Life draw point.
Exiting on the other side of the arena, enter the classroom on the right of the first screen to find another student who has a card key for you. You'll then need to return to where Raijin and Fujin where and climb the stairs to the third floor to find the next card key door. The party will leap off onto a familiar-looking athletic track; then follow the path to the center of the Garden. You'll notice a giant creature in the middle, but circle around him to find a save point to use first.
The creature is the Guardian Force, Cerberus. Apparently, in Galbadia Garden, they can just summon their GF and leave them out indefinitely. Anyway, make sure to steal a rare Spd-J Scroll from him and to use Dispel if he casts Triple on himself. Speaking of Triple, make sure to spend a good bit of time stocking up on this spell for your stat-junctioning, as it is one of the best spells for a myriad of stats. The boss, itself, has very low HP, so just use whatever you like outside of wind and lightning elements. For defeating Cerberus, you gain the Cerberus playing card and his services as your own, personal, Guardian Force. Since there are only three GFs in the game that have the ability to junction to a character's hit stat, another being Diablos, make sure you separate these two between characters. Also, it's a good idea to junction Cerberus and Pandemona separately to be able to junction to character's speed stat. You may consider using your newly-won Spd-J Scroll to teach the third character, without Cerberus or Pandemona, the Spd-J ability to one of their GFs. Eventually, you will gain a third GF that can learn Spd-J normally, but it will not be for quite some time. I recommend replacing any Speed junctions with you have with Triple and if you aren't using it already, Double for your Hit%.
Inside the atrium of the Garden, you will frequently encounter the Tri-Face monster from the MD levels of Balamb again. Now is the perfect time to farm Curse Spikes by stealing from them. Remember, they have Pain and Flare magic to stock, also. You will not be able to encounter Tri-Faces again for some time after this.
If your Siren GF is over level 100, at this point, I highly recommend farming 100 Curse Spikes to create a Dark Matter item. This will teach Quistis her best blue magic, Shockwave Pulsar. This limit break is one of only three attacks in the game that can break the damage limit of 9999. If you can't do this now, don't worry about it, we will need to farm Curse Spikes and Dark Matters for other items later on. If you do create a Dark Matter, make sure to pick up some additional Curse Spikes to use for weapon creation.
From the atrium, take the left-side path and into the classroom on the left. Here you'll find your last card key. With it, you can unlock the downward path that takes you back to the areas you've already been too, as well as the elevator leading to the Master's Room. To reach the elevator, return to the atrium, then back toward the athletic track, but take the stairs up; the elevator is to the left from the stairs.
Seifer and Edea are here, but you can hug the outside of the room to make it over to the save point before confronting them. Seifer, apparently, remembers everything from the orphanage, although it's never revealed how or when he was made aware. Depending on the party members you brought, Seifer will have different dialogue for each of them.
In battle, he's as easy as usual. Make sure to steal 8 Mega Phoenix items from him.
Edea will then meld into the floor below, which Squall, somehow, knows the auditorium is located underneath. Return down the elevator and take the path to the right-side of the stairs this time. Circle around, counter-clockwise, to a save point just before the auditorium. Use a Tent and prepare to face off with Sorceress Edea inside.
Edea's ready for your confrontation, are you?
Seifer's back, again...This time, however, although his HP is low, he can dish out some damage. Draw and cast Haste on your party and you'll either get a Hero or Holy War by stealing. When you strike him down, Edea will then step in.
Watch your GFs at the beginning of the battle with Edea; she will cast Maelstrom which will likely wipe out your GF's HP if your character has much higher HP than it. It's not that big of a deal, but you want your Guardian Forces to get the AP after the battle. Before you do anything else, draw the new Alexander GF from her. If you miss this chance, you will only have one other chance to acquire Alexander at the end of the game. Then, mug a Royal Crown item from her. The battle, itself, is pretty underwhelming for such a build-up. If you have positive status effects on your characters, she will spend time casting Dispel on them while you whittle down her HP. If you're concerned about the Curse (Can't use limit breaks) effect Maelstrom inflicts, you can draw and cast Esuna from her. Keep healed from Maelstrom and the battle should be won relatively easy.
A light will erupt from a defeated Edea and Rinoa, seemingly under a trance, will check on a defeated Seifer who will rise and walk off; followed by her collapsing. The disc comes to a close as Edea appears to have returned to the old Matron and frantic about the safety of Ellone.
