What's a good strategy for the Siege on Narshe segment?

The method I've always used is to position units to wall off Bannon after the battle starts;

[Bannon]                       [The save point that disappears]
[      ] [ Gau  ] 
[Terra ] [      ]

Team 1: Edgar, Sabin, Celes, Cyan
Team 2: Terra, Locke
Team 3: Gau

Team 1 auto battle:

Edgar running shoes, crossbow everything
Celes cast Ice/Ice all
Cyan genji glove attack
Sabin aura bolt everything

It's possible for Team 1 to kill everything fast enough you never see a separate battle. If Terra does get attacked, don't let the battle end with low hps make sure to cast cure before it's over. At this point you may wish to switch to Terra and fight a couple of the troopers that made it up there so you can access the menu after the battle.

Nothing should touch Gau, but there's a slim chance he'll solo a squad if it occurs. If Gau is in line with Terra and not offset there's more chances he'll be in a solo battle.

Kefka: Celes runic, Cyan dual attack, Sabin auro bolt continues, Edgar use items

The following strat assumes the player is playing the game casually and for the first time. Also, I have played the Woolsey translation over 20 times, and the GBA version once (Phantom Beasts? Yuck). Accordingly, the names of enemies and abilities might not line up perfectly.

Team 1: Edgar (with Runningshoes) and Terra

• Between Tools and Terra's spells, this team clears the trash quickly and efficiently, leaving maximum resources for...

Team 2: Celes, Locke, Cyan and Sabin

• This is your boss team. Cyan and Sabin are hell on wheels at single target damage and Celes's Runic forces Kefka to do his Magic Pot impression most turns. Locke is also there.

Team Rejects: Gau

• Don't get me wrong. Gau is really great later in the game, when he gets a larger repertoire of Rages and you get the Relics that really turn him on. For this fight, his biggest offensive tools are spells, which Celes will eat while Sephiroth's Role Model gleefully nukes your party. For now, he's the only one who has a shot of soloing any leakers (other than Edgar), so he's on Bannon Duty. You can swap him for Locke if you want, but make sure to pick Rages that don't cast spells, like Stray Cat, Stray Cat, or even Stray Cat in a pinch, and be aware that you can't use him to heal with items once he's raged.

Overall Strategy

Our goals are to defeat Kefka while preventing any enemy troops from reaching Bannon. If a party faints, they respawn at the top of the map. When the encounter starts, put team 2 directly "south," and Gau directly "east," of Bannon. Roll out with Team 1, but don't get cocky. Ideally Team 1 will take every non-boss fight. Definitely advance to contact, but once you kill a few groups head back north to catch anyone who slipped by you. The absolute safest strat keeps Team 2 in view at all times, if not Bannon himself, so you can move to intercept anyone who gets anywhere close. If someone slips by Team 1 and you have even a split second's doubt you can get back up there in time, switch to Team 2 and beat them back. There are a finite number of enemies, so once you run the Empire out of troops, you can take your time. It's worth noting that the Empire retreats as soon as Kefka is defeated, so you don't need to kill everyone. It's your first playthrough, do what you want. I'm also pretty sure the enemies can move when you're menuing outside of combat, so don't take too long looking at stuff. As long as you keep Team 1 in the top half and periodically check the top by switching to the other teams, you should be able to keep on top of the trash.

Trash Tactics

Edgar's Bio blaster will one shot the solider and Doberman enemies and severely damage the massive armored bulldogs (I remember them dying to the poison ticks on occasion), and with Haste he'll usually go first. Terra will be able to pick up the spare on the bulldogs easily, probably even with melee, but follow it up with an autocrossbow and you'll be golden. For the armor, use the autocrossbow and a multitarget fire spell, Fire 2 if you have it. Terra should still have plenty of MP to heal the occasional hit on the menu, but be quick and stay aggressive.

Oh $#!7, Gau's in combat!

Templar/Pterodactl should be able to deal with soldiers or soldiers with a dog, but he's pretty screwed if the Magitek catches him. Be ready to switch to team 2 and intercept the enemy when they beat Gau.

Hell's Rider

Gau can theoretically one shot the Hell's Rider with the Rhodox rage if Snare hits or the Commander Rage if he casts Break. It might be worth sending him in just to see what he can do, though he'll probably just get creamed by himself. Otherwise, Team 2 should do as much damage as possible. Locke should ideally have a weapon that works in the back row, because it switches you row when you hit it with an Attack. Cyan should use Dispatch, Sabin should Aurabolt (or Flame Dance, if you have it) and Celes should cast her strongest spells. You can steal a mithril vest from him, but he counters with a powerful attack that could easily KO Locke, and while the vest is good for this part of the game, it gets replaced relatively quickly. If resource management is a concern, just skip the steal. You can also skip the miniboss entierly if you move precisely enough, but then you can't find him on the Veldt for his underwhelming rage.

Kefka

For the Kefka fight, Locke melees, Cyan should use Dispatch (or whatever Sword Tech I is called in the non-Woolsey translation), and Sabin should use his strongest Blitz. I don't usually have Flame Dance by this point, so it's usually Aura Bolt (if you just got Flame Dance, Aurabolt might do more damage to Kefka, stuff scales weird sometimes). As for Celes, this fight is the most intensive micro of the entire game. Runic begins when Celes steps forward and you see the flash and hear the tinkle. It ends when a) Celes absorbs a spell or b) when you give Celes another command, including Runic (this was the case on the original SNES US cart. It may have changed in later releases or on ROMs). So, if you just mash Runic, there's a better than good chance that ol' Joker Senior will nail your party with a spell in the gap in coverage. So the best thing you can do with Celes in this fight is to hit Runic as soon as humanly possible on her first turn, and then skip Celes' turn (the GBA release says it's SELECT, but that really doesn't sound right) until she absorbs a spell, then immediately skip back to her and input another Runic. Don't be afraid to spend Elixirs if your characters get low, this is one of the harder fights in the game. Celes cannot outheal Kefka's magic damage at this point in the game, so don't try. If you miss a runic and take a bunch of damage, have Locke heal Celes while the other two heal themselves. Locke will have the lowest DPR at this point, so him missing an attack or three isn't a tragedy.

This part can be a little tricky, but with the right choices you should able to complete it without problems.

• Be sure that everybody has the best gear available to them! Check your inventory for equipment and relics. Everyone has to be at their best!
• Choose wisely your teams. Terra and Celes are the only two that can cast group healing, so be sure to keep them in different teams. In my experience, the teams that works the best are: Terra + Edgar; Celes + Locke; Cyan + Sabin + Gau. This may vary based on the power level of your characters.
• Often use your abilities that target multiple targets to quickly dispatch your enemies, such as Edgar's Bioblaster and Sabin's Rising Phoenix.
• Be aware of the Hell's Rider, the single boss before Kefka. He has a strong attack able to deal ~ 300hp damage, but he's weak against Fire. Use Sabin's Rising Phoenix will help take him down fast!
• Be sure to use Celes against Kefka! Her Runic ability will counter all Kefka strong magics, saving your team from the damage.

If nothing works out, try loading an earlier save and level up your character. Stronger character makes this part easier, of course.