Final Fantasy II/Mission to Salamand

Minwu the white mage
After you learn about the Mithril, you learn that you need to retrieve it somewhere from the northeast. Speak with Minwu to have him join your party and receive the Canoe required to cross rivers.

Minwu's attributes: ...

Towns of Paloom and Poft
If you have not done so already, make sure that each member of your party can cast black magic: some monsters are highly resistant to physical attacks and are more easily damaged using magic.

By talking to various characters, you can learn that the source of Mithril was to the north east. To re-establish it, you will need to reach Salmando, which is to the north-west of Poft. If you want to avoid walking, there are two shortcuts that can be used; a charter boat from Palm allows quick passage to Poft, and Cid has a charter Airship that allows travel to both Salmando and the Semite Cave.

Semite Cave
After arriving in Salmando, you will find that the empire arrested most of the town and placed them within the Semite cave. This cave is located to the southwest behind the mountain range, but can be reached directly using the airship.

If Minwu was knocked out and you don't have an exit spell, there is a chest to the north that contains the exit spell. However, this is guarded by a turtle that is more resistant to physical attacks. If you do not have Ice magic or you weren't inflicting enough damage per hit on the Sargent, you will have to exit the cave by walking.

BOSS: Sergeant
When you reach the bottom floor of the cave, head to the north and take the west door. If you characters aren't powerful enough to injure the sargent, switch to the magic spells.

Return to Altair and give the Mythril to Tobul. Next, talk to Hilda to learn about the Warship.