Head to Pharsalos Fort in Phokis; it's to the southeast in the Valley of the Nymph, north of the Chora of Delphi.

There are multiple ways to enter the fort. Up and over the walls, or to the east of the fort, a cave that will lead directly to the inside of the fort and underneath "Elpenor." The cave is unmarked and covered in vines.

Make your way to the top of the fort. "Elpenor" is standing at the top on a platform facing the middle of the fort in his usual blue robes. He is directly above the entrance of the cave. Except, spoilers, this guy isn't actually Elpenor. Still, kill him, get the note, and escape.

The real Elpenor is still in Phokis, except he's in the western part of the island in the Valley of the Snake near the Snake Temple. He's inside a cave.

Pass through Lalaia and Kephisos Spring. Follow the path until it's time to call Ikaros. Find the target and head up the mountain to the Snake Temple. There is a huge, winding snake skeleton wrapping around the ruins of a temple. Stick to the right of the ruins and up a path to a cave. There are several guards on the way up; take them out quietly.

There will be guards surrounding the cave entrance and Elpenor is inside. Like any mercenary fight, Elpenor is a tough tussle. Dodge his attacks and use abilities to take him down. Once he is defeated, talk to him. Confirm the kill and loot him for the next mission. Once this is done, the mission will be complete.