A bit of an easier solution than the existing video guide, which makes things more difficult than it needs to be.
Let's get a few things straight first:
-You do NOT need to kill the 3 soldiers in a single fight. In fact, I got it across multiple days, with my game and xbox being entirely shut off in between.
-Any shield-bearing enemy will do, so long as you don't break the shield prior to killing them. Shielded ceorls that put their shields on their back to switch to a two-handed weapon they picked up from the ground will still count, so they are your easiest targets due to their vulnerability to ranged attacks.
-Your in-game difficulty setting doesn't matter, you can get this achievement on easy (skald). The only thing that matters is that your power level is low enough that the enemy in question is displayed with the red skull next to their health bar. You can always lower your power level by unassigning your skill points.
-The most difficult thing is simply being on fire while dealing the finishing blow, since the fire effect disappears quickly. The best thing is to just stand on a perpetually burning area so you will constantly be reignited.
Setting a thatch rooftop on fire and climbing on top of it to kill enemies with your bow is probably the best way, since the rooftop burns for a really long time and gives you some room to move around.
In summary, the method is as follows:
-Go to a region of which the recommended power level is sufficiently high compared to your own power level that it displays with a red skull on the map. Lower your current power level by unassigning skill points if necessary.
-Find a village with thatch rooftops and shielded enemies. (The location in southern Hamtunscire used in Powerpyx's guide works, but there should be many villages in high level areas that work just as well)
-Do yourself a favor and (temporarily) switch the difficulty to easy, because you will likely take a bunch of damage whilst derping around to set yourself on fire.
-Weaken the shielded enemies first without killing them or breaking their shields. This can be done using abilities or by dodging around them and attacking from behind.
-Climb on top of a thatch roof, drop a torch on it to set it on fire, wait until you catch fire yourself and try to finish off the enemies with your bow. It is annoying to kill a shielded enemy at range while on fire, and some of your kills might get stolen by the fire damage over time your arrows will also inflict.
-Restock on arrows and rations, then rinse and repeat in the next village until the achievement pops.
NOTE: The achievement can still be glitchy/fussy even when done seemingly correctly and it may take more than 3 kills to get your achievement.