PS4 "Reverse Engineering" not working

I am unable to get the achievement "Reverse Engineering" to work. Killing the Gaia statues (corrupt) with the Gaia Spear does not trigger the achievement.

I have read you have to

  1. not have the spear
  2. cut off right arm
  3. pick up spear
  4. change weapon to spear
  5. kill Gaia with said spear

Is it only possible to get this when one does not yet have the Spear? If so I have to restart the game with a new character and play all the way to this point, hoping I don't die after having picked up the spear, just for this one achievement.

I have tried damaging the Gaia with the lowest tier weapon I have, but no matter which, when I proceed to cut of the right arm with the low tier weapon, it kills the Gaia.

After running around the area for 10 minutes I found one un-corrupted Gaia, that I was able to damage enough with a weak weapon so it did not die from cutting off the Spear arm. Then I switched to the Gaia spear I had found much earlier and finished it off. Achievement triggered.