I have encountered an issue with unlocking the achievement, Overdesign 2 in Assassins Creed Valhalla on Xbox One/Series X, but I have also read multiple cases of it not unlocking on other platforms.
The achievement requires you to kill 3 hard difficulty enemies without breaking their shields while on fire, I have viewed multiple achievement guides on the easiest way to unlock this achievement, as it does seem to be randomly obtainable depending on the persons game for what ever reason, but so far all strategies I have tried do not work, I de-levelled myself to be at least 100 levels below the region difficulty level (340 I believe in Hamtunscire and I was playing at around 240) the go to method is to light a roof on fire and to use the missile reversal technique to throw javelins back at the shielded enemies, multiple reports on reddit and youtube have reported this to be the easiest method for those who have been able to obtain it, some have had to fast travel out of the area to kill a different hard difficulty enemy elsewhere using the same technique in order for it to pop, I have attempted this method multiple times for hours across multiple days without any luck unlocking the achievement, despite successfully killing 3 or more hard difficulty shielded enemies while on fire without breaking their shields, I am also aware that a skull next to the enemy identifies their difficulty as being hard, which I made sure to take note of when attempting this achievement, I have also attempted hard resetting my console/clearing its cache and unplugging it at the wall for awhile, which also did not help.
I have also attempted it by letting the fire burn out on Eivor after a kill and then reigniting myself before attempting the next, I have maintained remaining in constant combat and killed between 5 to 6 shielded enemies in a single sitting while on fire and still no achievement, I have also attempted melee and ranged kills only instead of the missile reversal as well as hiding after each successful kill before moving on to the next, all while playing at at least 100 levels below the regions suggested difficulty of 340 to ensure that the enemies I need to kill have the skull next to them.
I also had a friend recover my gamertag onto his Xbox where he attempted to help me using the methods I have already listed, and committed a full day to trying to help unlock this achievement and even he came to the conclusion that it has to be bugged given that we were both doing every thing right and following the instructions listed on the achievement to a T with no success unlocking it.
I hope this report can be forwarded to the correct team who can work on a fix for this achievement, thanks.