Small section. You already should have Sabin, so no prep work. You’ll need to find Duncan’s house, which is found northeast of Narshe, which is on the north-central continent. You’ll see five trees; walk into the center one. Remember to bring Sabin.
Once you arrive, Duncan will come out of the house and proceed to teach Sabin how to use the final (and most lethal) Blitz - Phantom Rush . Normally, you’d have to wait until Level 70 for this magical (yes, MAGICAL) attack of insane power. Enjoy.