I have a much easier method for this Achievement.
You'll want to unlock the skill called "Missile Reversal". You'll find it down in the Raven Tree (Assassination Tree / Yellow Tree).
You'll want to make sure that you're low enough level so that the enemies in the area have a red skull. You'll want to head to the area Werham in South West Hamtunscire and proceed to the northern entrance as shown in Powerpyx's guide. This area is great as there's a lot of Standard Bearers there. Hamtunscire is a Level 340 area, so make sure you're no higher than Power Level 300.
Next, all you need to do is aggravate the Standard Bearers, then go to a nearby thatched roof and set it on fire. Stand on the thatched roof and the Standard Bearers will start throwing spears at you. Simply press the LB when the Spear is about to hit you and Eivor will grab it and throw it back, doing a considerable amount of damage whilst being on fire from the thatched roof.
It really is that simple.
Credit to @Danny Bizz & @Kleptojack
Some people have had success doing this achievement as 3 separate fights (Kill one, go hide, kill another etc.) instead of killing 3 of them in one go. Other people have had to kill more than 3 (anywhere up to 10 or so). This Achievement is definitely temperamental so keep persevering.