UPDATE: As of 7/5/2017, there is an event going on and there seem to be Volge Huntmasters at most, if not all minor arkfall impact sites. The event won't last forever and who knows what will happen after that, so hurry up and nab this achievement before it's gone forever. I will update this once it becomes unavailable again. Hurry up and good luck to all.
As of 9/5/2016, I believe the chances of actually finding a Minor Arkfall Impact with the Volge Huntmaster inside is below 1%. I have called down numerous minor arkfalls hoping to finally get the Volge, but it has always been the Grid.
There is still a slight chance of possibly spawning one of these (very slight chance) but then landing the killing blow on him just makes your chances even that more unlikely. I am uncertain if it has been removed from the game or not.
You can try having a group of high EGO players bring up the Threat Level to 10 and then dropping down an ark spike, but you still probably will only get the Grid to spawn. Even if that is the case, only 1 of you will get the achievement as you need to land the killing blow on it. You also have to worry about randoms stealing the kill from you.
If you still want to try this, here are some things to know. You will need 1 ark spike which you get from the daily and weekly rewards, or you can purchase one from most vendors for 10,000 Scrip. Once you have an ark spike, search on your map for a small red circle (kind of looks like a regular arkfall site). It will either be a Minor Arkfall Impact or a Major Arkfall Impact. You are looking for Minor arkfall impacts. Head to one of these by setting a marker over the impact site. You should see 1 blue ring. (If it's 4 then it's a Major Arkfall site and you will not get the Huntmaster). Walk up to the blue ring and Hold X to drop down your 1 ark spike.
If there is already enemies around and a large structure in the ground, it means someone else called it down first. What you want to do either way is kill all the enemies. Once you kill all the enemies, you will see 4 blue rings surrounding the large structure. Walk inside one of these rings and Hold X to enter the structure. You must now fight off a lot of enemies. If you're lucky the Huntmaster will be inside. Read the other solutions for how to get the kill shot on him, if you are worried you won't kill it. If you are able to get the kill shot, the achievement will unlock either right away or once you get the results screen for finishing the arkfall.
How to know if it is not the Huntmaster: Firstly, I have noticed that mutants spawn almost every time and then the Grid. If the Grid show up, you know that it will not be the Huntmaster so don't bother entering the structure. If it is anything other than mutants spawning first, I would enter just to be safe (even if you get more grid). I have only seen Mutants and Grid, so anything different don't risk the chance of missing out.
So in short, call down the arkfall. If it's Mutants/Grid, just leave because it's not worth the wasted time. If you get any other enemies (especially Volge), enter the structure and hope it's the Huntmaster. Good luck to everyone who is attempting this and hopefully they will bring it back if they actually did remove it. Or at least increase the chances of it to spawn.
I added a video below on what a successful Huntmaster Minor Arkfall should look like and what the Huntmaster looks like.
Video credit - Mr Jason Well