This achievement requires you to kill a Behemoth tank. You can find one in Behemoth Defense Area.
Bar far the easiest one I was able to do is in the Fen Bog area, west of Broughton Channel Bivouac, south of Longford Farm, north-east of Holmwood Mangrove Bivouac, right dead in the middle of Homwood Mangrove letters on the map. Now I was item level 185 when I did this so I’m not sure if this location has a level recommendation. Now for me this location had a turret and the tank was the one that shoots out toxic gas. I don’t know if this is random or not so that’s why I’m putting this out there. If you haven’t put the two together basically hack the turret and go to town on the tank. My life went down to half when I destroyed the tank. I quickly had to exit the turret and run away from the gas that was showered on me and use a Syringe to make sure I didn’t die before I got out of the gas radius. The achievement should have popped the moment you killed the tank.
I know this game has been out for a good while but I picked this game up for a cheap price and noticed that the only guide wasn’t that helpful. So I hope this one was a little bit easier. Good luck.