Quickly obtainable in a hot seat game where you play as both players.
Play hot seat
Duel map
Any map type (Inland sea works well)
Online speed
Ancient Start
No barbarians
Set player one to Harald Hardrada. Player two can be any civ.
As Harald, place your first city on a coast. If you don't see a coast nearby, a restart may be faster than scouting one out. Prioritize getting out a Viking Longship.
As Civ #2, scout out player one and take either your starting settler or a quick second settler to the coastline near Harald.
Declare war between the two civs. With the settler of Civ #2 on a land coastal tile, have Harald's longship pillage the tile the settler is on. This will pop the achievement for you.