When farming Plague Walls you will encounter more than enough Ferals to get this achievement, so it’s much more efficient to not actively seek them out during the rest of your play through given how irritating they can be to kill generally.
Plague Busters will drop them very quickly, so either have a big supply of those for both the walls and the mobs (to avoid the bother of conventional killing) or save scum the final wall to have “unlimited” resources and easily get this achievement and the Blood Jugs achievement at the end.
To easily farm the final wall, pull a vehicle up to the awning outside of the store to the left of the wall if you’re facing it (this is outside of the Plague Wall encounter area and the mobs cannot reach you here). Climb onto the awning and throw a Plague Buster to commence the takedown, then follow up the throw with Fuel Bombs/Molotovs for AoE damage. As the wall gets lower on health, Ferals and Juggernauts are more likely to appear - it is possible at this point to enter the area and fight them head-on for simpler kills. When you’re done, finish off the wall and as soon as the ‘Plague Wall Destroyed’ message displays navigate to the game save file on your Xbox (through Manage Game & Add-ons) and delete your save FROM YOUR XBOX ONLY. This will auto-quit your game and when you reload the game and backup from your cloud save you’ll be back at your last auto save (typically part-way through the fight) with all of your equipment/ammo etc.
I got auto save messages at many different points throughout messing around with this technique but it always brought me back to after the first time I actually defeated the wall and played out the final scene no matter how long I spent/how much progress I made on achievements. That being said, short bursts of gameplay are probably safest if you want to ensure you keep your supplies.