If you don't have this achievement before the final part of the game, you'll hit an unfortunate snag - environmental objects simply stop respawning after a certain point. You'll end up clearing out areas of all the items and they won't come back.
I searched and found a solution if you don't want to get the achievement on a new save. All credit to this post:
https://www.playstationtrophies.org/forum/538598-post21.htmlTake a Serpent Hole to Dry Road. Go out of the cave and climb the demon growth. Ahead are a couple of lamp posts, grab one and keep going up the road to find 2 zombies. Kill them but make sure to not throw/drop/break the lamppost - keep it in your hands. Save your game then load that save. You'll be back at Vulgrim and the lampposts will still be there, repeat the steps above to get 2 kills each time. Slow, but it works.