Once you enter the town of Mobliz, ruined by Kefka’s Light of Judgment, some dogs will begin to bark at you, warning the kid within. The kid comes out, then re-enters the house.
First, head to the ruined house west of the kid’s house and examine one of the nearby barrels for a Phoenix Down , then head to the abandoned Relic shop to the north. Sleep in the bed in the back for free healing, then go into the house the kid from earlier went into.
Inside, head downstairs to find a bunch of kids, and Duane and Katarin (minor characters, but named from before). I doubt that they made them all, though. Anyhow, apparently, Terra will also be here, refusing to leave.
So, Terra, our (or at least MY) most valuable character, isn’t going to join our fight. Disappointing, but we can’t really just force her along. So just leave. As you do, a kid comes in screaming about that Humbaba, some demon unsealed by the planet’s ruining, is coming this way.
Terra alone goes out to fight it (and why not Celes or Sabin?). Terra is meant to lose this first fight against Humbaba, so just let it smack her around until she is KO’ed, then Celes and Sabin take over.
Humbaba can use a basic attack, Solar Plexus (which is three times as powerful), 1,000 Needles (1,000 damage on target), Thundara, and Thundaga .
Begin the fight by using the Kirin and Zona Seeker Espers to provide Regen and upped Magic Defense, respectively. Have Celes use Bio or a "-ra" spell (except Thundara) until some healing is needed, which should be dealt with via a party-wide Cura.
Sabin should focus on using whatever Blitz you tend to prefer, or, when using Venom Claws as a weapon, a basic Attack. Of course, if he has Bio, try that and see if its damage exceeds his physical damage.
After that battle, Terra again says she won’t join you, using this fight as an example. As you leave, you’ll receive the Magicite for the Esper of Fenrir . Fenrir is moreso based on the monstrous wolf of Loki’s in Norse myth.
He won’t provide an offense, but rather make you invulnerable for a few hits. So, I guess we’re done here. Heal up in the back of the ex-Relic shop, then return to the overworld. Go to the "head" of the Serpent Trench in the far north, Nikeah.