The hardest achievement in the DLC. You first need to complete the story to be able to summon arkfalls and then you can look at your map for small red markers indicating a potential Major arkfall site. Go there and use your spikes to summon/help to summon the arkfall. Other players should help out but you get one spike a day and can buy more for 10,000 scrip from vendors.
Once summoned you will need to defeat waves of foes outside the arkfall, and then some more Volge inside. After that you will drop down to face the Warmaster. Most of the regular players have gotten pretty good at beating him nearly every time, and thanks to a recent patch with the 7th Legion DLC, everyone in the room when he dies will get the achievement (assuming you did some damage). However, he seems to have been buffed even more and takes serious amounts of damage so if you don't have a full crew that know what they are doing then this could take a few tries. Here is a great guide to taking him down: