Head back to Mobliz. If you do some investigating, it seems that Katarin and Terra are missing. Go into the westernmost house and behind the bookcase downstairs to find Terra and Katarin. After a scene there, it becomes apparent the Humbaba’s back. That thing’s getting almost as annoying as Ultros. Go outside so we can kill it.
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 some defenses. Then simply batter the beast with some Poison/Bio spells until it eventually dies. It is a rather simple concept.
Eventually, most of your party will be blown away and Terra will join the battle. Keep up the above strategy with the Trance Terra and you’ll win fairly easily. Her Attack alone deals over 1,000 damage easily.
After the battle, watch the interesting scene in which Terra becomes a member of your party once more.