South Figaro Cave Redux
Locke and Celes will find the South Figaro Cave has changed quite a bit since your first trip. There are all new, tougher enemies, and a brand new boss. On the upside, two of the three chests have improved contents, and the scenario ends as soon as you leave.
None of the monsters here is vulnerable to Ice, and all but the Trilobiters have low Defense. You should be able to take out one of them or a Primordite with a single Ice spell, or a group with two. Gold Bears have a lot of HP and the nastiest special attack you've seen yet, Gouge, which deals two and a half times the damage of a normal attack. Also, if Celes has happened to learn Imp yet, don't use it on them, as it will only make them stronger.
The other two enemies are less of a problem. Primordites can use Numblade to inflict Stop but are otherwise pretty easy to kill. Trilobiters have much better physical defense due to their automatic Safe status, and their special attack PoisonBarb (predictably) inflicts Poison. Any spare Star Pendants or the Ribbon you may have found can prevent this, or Celes's Antdot spell will cure it. Groups of Trilobiters are a good candidate for mass-targeted Ice spells.
Modified Treasure
If you've been strategically skipping chests earlier in the game, it's finally going to start paying off. The Tincture in the eastern passage hasn't changed, but the other two treasures have if you didn't collect them already. The Fenix Down in the southwest, near what is (this time) the entrance to the cave, is now an X-Potion, and won't change again. There is no reason not to grab it now if you haven't already.
The Tincture in the upper cavern area is an interesting story. it is now a Thunder Rod, a weapon which no one in the party will be able to equip for quite some time. However, it does give you a one-time shot of super-powered Bolt 2. This is particularly useful in the immediate future, since it will take out the boss ahead in one shot. On the other hand, if you leave this chest alone now, it will become a useful Hero Ring in the World of Ruin.
TunnelArmr
During your trek through the cave, you'll notice a grinding sound every so often. That's TunnelArmr, who will attack right before you exit the area. This boss really has everything going against it: it reveals its presence early, it attacks you immediately after a recovery spring, it can be killed immediately by the Thunder Rod in the cave, and Celes can absorb most of its attacks with Runic.
At the beginning of the fight, TunnelArmr can use Bolt, Fire, and Poison as well as make normal attacks or use its double-damage Drill special. Celes can and should absorb all of the spells with Runic, because they deal well over 100 damage with each casting otherwise. Locke will have to do almost all of the actual damage here, so give him any relics that will help with that. TunnelArmr will sometimes counter normal attacks with an attack of its own, so don't hesitate to have Locke use Potions to heal without interrupting Celes's Runic use. Locke can also take some time to Steal, potentially earning a second Air Lancet (and not incurring a counterattack).
Once TunnelArmr loses 70% of its 1,300 HP, it will attack slightly more often and add Tek Laser to its combat options. Fortunately, this is still an ability that Runic can absorb, so the strategy doesn't really change. TunnelArmr is faster than Celes, so expect an attack or two to get through as the battle continues. Once you see Tek Laser, it might make sense to go all-in on offense to finish it off.
You'll earn an Elixir with the victory, and the scenario will immediately end. Unfortunately, you won't have the opportunity to unequip your party members so their gear can be used in the other scenarios. If you want to do that, you'll have to do it beforehand. If you use the Thunder Rod to take out TunnelArmr, this shouldn't be a problem.
Use one of the links below to proceed to the guide for the next scenario: