Final Fantasy VI -> Sidequests, Terra
If you followed my chapter 11 guide, you'll have already completed the first part of this side quest by fighting Phunbaba off in Mobliz. Return there to engage him in another fight and have Terra join your party.
For reference, Mobliz is on the right side of the giant U at the middle right portion of the map. Sleep in the relic shop in the back of town to rest up, enter the building with the kids to learn about Katarin, then enter the far left building and go behind the stairway to meet up with Terra. Phunbaba will attack, and you'll have to fight him.
Phunbaba is weak against poison/bio type spells, and has about 26000 HP. Fight him with your regular party for a while, then he will sneeze away your two middle party members. Terra will join you in permanent Esper form until you beat him. At the end of the sequence Terra will be in your party and you'll be free to go.