Protect Banon during the river battles.
Walkthrough
Before leaving the hideout, speak to the available party members and inspect the rooms for supplies. When the warning arrives, the group departs on a raft with Banon. Check equipment and save before stepping onto the river.
Banon is essential to this sequence: a defeat for him ends the attempt. Use his Health command to restore the group repeatedly and protect him from being overwhelmed. The river has junctions; follow the current until the octopus encounter.
Ultros separates 
Sabin from the others after the fight. On the scenario selection screen, finish all three paths. The selected group’s possessions and abilities differ, so save whenever the game returns you to the selection point.
Checkpoint
Banon’s Health command is a repeatable party heal and is particularly useful here. The encounter with Ultros is required. After the raft sequence, the three scenario choices are a way to choose order, not mutually exclusive story branches: finish each to reunite everyone at Narshe.