I tried to reply individually to people who were having this problem but it flagged it as spam? So I'll just make a whole new post and hope that works. I literally made this account just to help people with this problem so I'm not sure how to use the forums properly but anyway, here's the solution.

1. Make sure you are under level 300 (I was level 250)
2. Make sure you have fire resistance skills disabled in your skill tree (these are scattered around in the red bear part)
3. Make sure you have the Missile Reversal skill in your skill tree (In the yellow raven part right at the bottom, just left of the bottom blue wolf part)
4. Travel to Werham, in the South of Hamtunscire
5. Make a manual save (just in case you mess something up)
6. At the north of Werham (by the gate) are two shield bearers. Just behind them is a random enemy (this enemy is not important for the trophy) stood underneath a stall with a straw roof. Just behind that is a third shield bearer. There is a fourth shield bearer by the church in Werham.
7. Stand on top of the stall with the straw roof mentioned earlier (with the enemy underneath) and shoot one of the shield bearers with your bow to trigger combat (if you are not already in combat - DO NOT KILL THE SHIELD BEARERS YET)
8. Set the roof you're stood on, on fire with a torch (sometimes shield bearers will throw fire which will work too). This should constantly set you on fire.
9. DO NOT SHOOT ANY ENEMIES. Instead, wait for the shield bearers to launch their spears at you, and when the spear gets close, use missile reversal to kill them. This should one shot them, or get them very low.
10. Use missile reversal to kill the two shield bearers at the gate, and the third shield bearer close by. DO NOT LEAVE THE ROOF - standing on the roof means you're constantly on fire when killing the enemies.
11. STAY IN COMBAT, DO NOT GO ANONYMOUS. Go to the church (it doesn't matter if you're no longer on fire at this point) and kill the fourth shield bearer near it, using the same method (stand on a straw roof, set it on fire/set yourself on fire, use missile reversal to kill the shield bearer).
12. Travel to Fearnhamme. It is to the East of Hamtunscire. (I fast travelled to the fast travel point just north of Fearnhamme and then travelled by horse to reach it)
13. There will be a group of enemies in Fearnhamme, including one shield bearer. Stand on a nearby straw roof, and repeat what you did for the previous enemies (stand on the fire roof, then use missile reversal to kill the shield bearer).
14. The trophy/achievement should unlock after this. 

On PS4/PS5, missile reversal works by pressing L1 when the spear flung at you gets close, not sure what it is for other controllers/keyboards or whatever. I spent 2 hours trying to get this trophy and it was so frustrating when I did everything right. So when I followed this method and it worked, I knew I had to tell everyone.

Some of the steps might not be necessary (such as disabling fire resistance skills) but I did all the steps and they all worked so I just wrote down everything I did just in case.

Hope this helps!

@bobtobobb5397 I had problem with this one as I finished the game completely, and I could not trigger conflict at all (might be a bug as this was my last trophy).
However, even without conflict triggered trophy popped up after killing 5-6 of any guard with shield without breaking them. And it was not in a row, they were stationed randomly around the village.

Basically... I set my power level to 290, lighted the roof on fire and stood on it while shooting any of guards with the shield. (it took about 5-6 of them)

@bobtobobb5397

After a trip to Norway following a different suggestion then hours in Werham trying the first part this suggestion which is shown on many videos, following up with Faernhamme worked first time. I was quite amazed after so long trying. Brilliant stuff bobtobobb5397. Thanks!

@bobtobobb5397
Ok this is a noob question - how do you disable fire resistance on a ps4? I am probably being incredibly dense here.

@bobtobobb5397 OMG!!! Thank you so much for this post!!! I have beaten the game, dropped my level down from 400 to 265 and have been farming at Werham for hours upon hours. I was so afraid I would have to restart this game and try it from the beginning, but then I was doing a search here on the forums (all the google results did not work for me) and my very first attempt following your exact methods the trophy finally popped!!! Thank you so much for this!! I wish I could rate it higher!!!

@bobtobobb5397 YES! This worked for me when other videos did not. Perhaps traveling was key. I did not deactivate all my fire res. Appreciate this!

OMG thank you so much!!

I tried many things before this! (at least 20 tries in Werham).

Following your advices it worked perfectly! (from the start, I was down to level 270 with fire resistance ON).

First I pulled and killed 5 shield bearers in Werham (for this I had to set fire to two building, one near the N entrance, the other near the church).

Then I quick travelled near Fearhamme, pulled the standard bear along with the archers, then set the building on fire. Interestingly the trophy was not provided by the kill of the shield bearer but by the kill of the last archer.

Thanks again!

@bobtobobb5397 - Absolute legend, thanks v much, worked perfectly for me (didn't disable fire resistance but did need to be below 290 skill level to see red skulls).

Don't suppose you have a solution for the hanging key glitch in Jotunheim?

@bobtobobb5397

Tried your solution but it didn't work the first time. So when I was going to retry it at Werham, upon killing the first Standard Bearer. The trophy popped out. Thanks man!

@chrisbaker1972 - The development team are aware of the floating key in Jotunheim and are currently investigating this further. We've had a report that player was able to collect the key by shooting at the key from the other side of the river, between 2 rocks. You could give this a go and see if that works for you!

Thanks for the reply @Ubi-Borealis, at this point (227 hours into Valhalla) I'll try anything. It would be good if the next patch could fix this issue (maybe allow you to just press Triangle as you fall, similar to how you collect some of the notes when diving off a building).... I loved this game but this glitch is soooooo annoying.

@Ubi-Borealis - I tried standing between lots of different rocks on the opposite bank but no luck, the key is never targetted and I spent the whole evening either reloading or looking for arrow top ups to keep trying. Aaaaaaarrrrrrggggggghhhhhh

@ubi-borealis Thanks, will do, I just want the Completionist trophy asap

@bobtobobb5397 You are the frickin man! You have no idea how many hours for the past 3 months I've spent trying to unlock this. Going to Fearnhamme didn't trigger it for me because at first I only removed all fire resistance skills and was still at power level 350. Being under power level 300 was what did it for me and it triggered at Werham right after the 3rd soldier kill. I would still remove all fire resistance skills and runes and load up on armor and health because you will def need it after nerfing yourself down to power level 299 or under. Still go to Fearnhamme if the trophy doesn't unlock at Werham, this achievement is really bugged and you have to have all the stars aligned to get it right.

The only requirement is the difference between player level and NPC level - NPCs must be seen as red skulls.
Apart from that, you can kill ANY three soldiers regardless of the type, as long as each one has a shield, and not necessarily in the same combat.

I killed the two Standard Bearers at the above mentioned gate in Werham, Hamptunscire while burning on the roof of the house right next to the gate and, because I had killed all others around before that, I had to go down from the roof, out of burning, out of combat and pull a third ordinary soldier with a shield who was closer to the northern wall. Lowered his health to one shot, went back to the same burning house, climbed on the roof and fired that one final shot.