So at this particular moment in time, there is a glitch which enables God mode making this achievement 8264284 times easier... (If it gets patched i will delete this.)
(I know this isn't the intended way and some people may not like it but this is up for those who struggle with this or other bosses.)
So what you need to do is go to a place where you can play orlog (The dice game) and this is available after you beat story arcs and your people are settled there.
You will get the chance to choose heads or tails, pick one AND AS SOON AS YOU SEE THE COIN FLIP ON SCREEN, PAUSE AND GO TO YOUR WORLD MAP!!
Go to literally any viewpoint in the world and fast travel there.
When you load up, if your hud has disappeared that means it's worked!
So now, no enemy can kill you or even acknowledge your presence (Ignorant shits haha)
Anyway, next head all the way north to Hamtunscire.
On the map there is a main village and just below that with a viewpoint is where you need to go to get this.
Make sure your level is at least 40 levels below this area (Making sure the enemies have a red skull next to their name).
Obviously legit, you would need to weaken them first, then they can throw fire bombs at you to get on fire to get the final kill in.
With god mode on, they walk around but don't attack, so you can either set something on fire, whistle next to an enemy to lure them to your location and with one or two heavy attacks they die.
The best method seems to be, plenty of thatched (hay) roofs about.. find an enemy with a shield (this can be any enemy btw, as long as they have a shield).. set it on fire so your constantly burning, then either jump down and heavy hit them, or keep hitting them with your bow after weakening them first.
Either way, this achievement for the time being just got a little easier.
Apologies for the long explanation, so here's a potato... 🥔