Back in Vector
With Number 128 defeated, you'll find yourself back in Vector via the train tracks you followed toward the MagiTek Factory earlier. There is a brand new save point to the north, and you should use it since yet another boss battle is coming up. There are also now random encounters in Vector featuring a new enemy, the Chaser.
When you finish a lone Chasers off, it is replaced by three Trappers. If on the other hand the Chaser was accompanied by Pipsqueaks, it will re-summon them if they are defeated while the Chaser remains. Chasers have a lot of HP and a high resistance to physical attacks, but Thunder Magic is extremely effective against them. They will use the powerful Plasma Ability as well as Dischord and Tek Laser. They can also give themselves Shell with Program 17, making it that much more difficult to go through their 1,202 HP. There are no surprises beyond the Chasers during the escape. You'll run into groups of four Pipsqueaks or Commandos, but you should have no issues defeating them at this point. You are running a three-person party at the moment, though, so be somewhat cautious.
The Cranes
You'll meet Setzer once you get closer to town, which begins a cutscene. Kefka launches two large cranes to catch the Blackjack as it tries to fly away. Setzer joins the party for this fight. You'll find yourself on the Deck of the Blackjack, caught between 2 Cranes. They are similar in some ways, but each is based on a different element. The Left Crane is Thunder-based, while the Right Crane is Fire-based. Both have similar attack routines: both can use normal attacks, a nastier Iron Ball attack, or a tier-2 attack spell (Bolt 2/Fire 2). When either Crane is destroyed, the other one will use TekBarrier to give itself Reflect and Safe. Both are vulnerable to Water, making your new Bismark Esper a powerful summon.
Outside of the basics, the 2 Cranes have some important differences. Both absorb the Element they use offensively, but the Right Crane is vulnerable to Thunder, and has 500 more HP to make up for this. Further, if you use the Element they absorb on them, each Crane will charge up in addition to healing HP. After three charges, they'll unleash a nasty attack, either Giga Volt or Fire 3 depending on the Crane. Simple enough, you might say, just don't attack them with the wrong element.
What complicates this fight is that, each time you attack either Crane three times, it will intentionally charge up the opposite Crane one level. A few missteps, and this could spell disaster for the party. Your best bet, aside from using powerful Water attacks, is to focus on one Crane at a time using powerful attacks that will burn through their modest HP before they have a chance to charge. I recommend taking out the Right Crane first with Thunder, then finishing off the Left Crane with physical attacks.
As a side note, Setzer is not terribly useful in this battle. He knows no Spells and has mediocre gear, so he probably won't add much beyond random effects with Slot. Your established party will be carrying the weight here.
The Esper World
Once the fight is won, you'll automatically fly the Blackjack to Zozo to check up on Terra. One of your Espers—specifically Maduin—will wake her up. You'll find yourself in the Esper world controlling Maduin in the past.
Your first order of business is to go to the Gate, where you find a human woman named Madonna. You can explore the area and speak to the other Espers first if you wish. Maduin will bring Madonna to the Elder's Home. When Maduin wakes up, head back to the Gate to find Madonna.
Several scenes will play out, and the time advances two years. Speak to the Esper blocking the door, then follow Madonna (once again) to the Gate. This will trigger the final cutscenes in the Esper World, after which the game returns to the party in Zozo. Terra is now awake and aware of her nature. She rejoins the party, and now has access to the Morph Command.
The party decides to return to Narshe, and Setzer gives you control of the Blackjack. Finally, you can explore the World of Balance at your leisure! You can change party members by speaking to them on the Blackjack, and the party members you left in Narshe are available as well as Terra, Setzer, and everyone who was in your party previously (except Celes and Shadow).