The general badassery of Isaac and friends' exploits over the course of the story practically makes them living legends in the sequel. Exploits including saving a town from a tree curse, saving the spirit that cursed them, protecting a boat of civilians and sailors from a Kraken, saving a powerful emperor, and much more.
The djinn summons are a spectacle of destruction to watch. Notable mentions are:
Playing for the first time? Good for you! Found four Venus Djinni yet? Great! Go unset them all and summon Judgement.
Meteor calls forth...well a meteor. A giant space rock burning through the atmosphere to crush your opponents and also wipe out the dinosaurs.
Tiamat is a rather simple summon, but there's something cool about a big ass dragon dropping out of nowhere in front of your enemies and blasting them with a breath of fire.
Atalanta drops a Rain of Arrows down onto her opponents from the heavens. The cacophony of your enemies being speared repeatedly is immensely satisfying.
Rameses, a cyborg Pharoh robot thing that drops several rocks onto its opponent before blasting them in the face with a Rocket Punch.
Isaac competes in Colosso, an annual tournament where the strongest fighters in the world fight for glory. He's forced to go it alone for the first and only time in the game and can end up winning the entire thing. Of course he can also choose to get some help from his friends on the sidelines...
Breaking into Lunpa to rescue Hammet. The dungeon is a Stealth-Based Mission where Isaac and the others have to outwit the greatest thieves on the continent. Then when Dodonpa sics his giant monster on them, the group beats that thing too and then walks out with Hammet.
Beating Deadbeard apparently earns Isaac the title of being the lord of all pirates according to some NPC dialogue. Too bad nothing else comes of it.
Felix standing up to Saturos and Menardi at the top of the Venus Lighthouse to convince them to let him take Sheba back to the ship, despite being very out matched and standing no chance against them.
Followed by Isaac and the party beating Saturos and Menardi and then after they fuse into the Fusion Dragon, beating them again.
At the end of Golden Sun, due to an earthquake, Sheba falls off of the Venus Lighthouse, most people would let out a Big "NO!" and sob as they failed to save someone. Felix jumps off right after her to save her and succeeds without any injuries.
Golden Sun: The Lost Age
Jenna fights her way through a bunch of armed men on her escape from Venus Lighthouse. Despite being a Damsel in Distress for the entire first game, Jenna is the first character you play as and it's awesome. She even gets her own unique battle theme that you can listen to later in the game if you remove Isaac and Felix from the party and make her lead.
Alex's Smug Snake nature as he effortlessly kicks the crap out of Tolbi's soldiers. Keep in mind it's been established that all of Tolbi's soldiers are Colosso winners, meaning Alex beats the crap out of an army of the world's strongest men on his own. This was the first of many of Alex's Magnificent Bastard moments in the sequel.
In Contigo, Alex tells Kraden that Lord Babi has died, which would then mean that Kraden no longer has any reason to continue adventuring with Felix and his crew and could therefore choose to go home if he so desires. The old scholar, despite his initial grief, immediately chooses to continue onward and stick by your group, especially since he has now started theorizing that Alchemy's seal is slowly destroying the world.
Beating Poseidon, something no one had ever done before.
Isaac's group falls into a trap at Jupiter Lighthouse by Karst and Agatio that puts Garet in mortal peril and Mia in a position where she can't help. Isaac not only immediately recognizes who Karst is related to by her appearance, but essentially tells them he doesn't give a crap about her want for her revenge and to get out of his way so he can save Garet. Isaac and Ivan end up fighting Karst and Agatio two on two and lose, but even afterwards their assailants comment on how tough Isaac was even fighting without a Mercury Adept's help. Furthermore they weren't actually able to finish him off and have to stand down when Felix shows up because they don't think they can take his whole group as they are now. To say that again, Isaac was strong enough to give the main villains a run for their money while essentially fighting at half strength and without someone that had an elemental advantage.
Piers knowing that after Felix lights the Jupiter Lighthouse, Karst and Agatio will likely try something. He's proven right as Karst and Agatio try to kill them and the party can easily hold their own as Jenna and Sheba show up to help and Karst and Agatio get annoyed because their plan is backfiring. This can end with the party defeating both Karst and Agatio.
Shaman Village is a series of moments of awesome for Sheba. Just like how Jenna completely shook off her role as a damsel and became a Magic Knight badass, Sheba, in most spectacular fashion, makes up for all her scenes of distress from the first game and then some.
First, she calls out Moapa and his village for their flagrant sexism when they're utterly baffled that a woman could be their newest hero of legend.
Sheba: What, are you saying I can't be the chosen hero?
Moapa: The heroes have always been men, ever since the time of Hoabna. It is the way.
Sheba: I don't like your attitude, mister! I can be every bit as heroic as some guy!
Then, when the party coerces Moapa to give them the test to prove Sheba is the hero, she immediately passes the first leg of the challenge in such a manner that Moapa is momentarily floored and is forced to let Sheba take the main test.
By the time Sheba and her friends get to the top of Trial Road, working as a team, even Moapa and his men fully admit that they are actually impressed in Sheba, and give her the final part of the test; a climatic battle that ends with Moapa finally acknowledging that Sheba is the new hero of Shaman Village.
Agatio calling out Alex for thinking Agatio doesn't know he's being used as a simple strong arm. Agatio is aware, and was the first character to ever verbally suppress Alex, just by calling him out on his pointless grandstanding and duplicitous nature. Agatio knows he's being used, he just doesn't care because, at the end of the day, whatever plan Alex has won't change the fact that by lighting the Lighthouses to achieve his goals, Alex will need to let the heroes win. And Agatio is entirely correct. Alex immediately changes the subject afterwards.
Traversing the Mars Lighthouse. Blasting ice with fire was never so satisfying. Saving the world while doing so? Priceless.
The sheer determination of the party to light Mars Lighthouse, despite the Wise One's interference.