Achievement says. Don't allow all the shadows to gather. We have killed some of the shadows on the ground level and left some alive with no success. Killing all of them gets you None Together now. Does this achievement really mean Dont kill any of the shadows on the ground level or does it mean leave a shade up top alive?
Just let one of the downstairs aura dudes get to the aura. Just one. After that kill the rest entire event and BOOM achievement.
Yeah, the phrasing of the description for this one needed a little emphasis: "Don't allow all the shadows to gather." So you allow one, then kill the rest. That also means spending some time just sitting around with Doomshade active, since shadows are only active during Doomshade phase. It takes some time for the very slow shadow to reach the aura.