Taken from my solution on TrueAchievements:
Here’s a method that requires no glitching or real combat. You can do this at a level commensurate with your arrival in England.
Once you’ve reached England, fast travel back to Norway. Then, fast travel or sail to Alrekstad. These soldiers will all be level 280 or so and will not attack you no matter what you do. You can whittle down the health of any soldier that has a shield with your bow, sneak attack, or regular attack. Once you’ve gotten them down to a sliver, go and stand on one of the braziers until you are fully aflame. Once you’re ablaze, attack the target until they’re dead. Provided you still look like Ghost Rider, it’ll count.
Rinse and repeat until you have the three.