Welcome the Void

About forty seconds into the fight the boss will target a random player and cast [spell=244433] which will knock that player backwards into the void realm.
Now that player will want to fill up their energy bar, attack [npc=126283] and then use the extra action button [spell=244621] near him so that both of them will get sent back to the normal realm.
From there you just kill [npc=126283], then [npc=122313] and there's your achievement.

Other achievements:
Let it All Out
Darker Side

As of patch 8.0.1 this achievement appears to be broken, and it is unable to be soloed. The boss simply doesn't cast umbral shift.

09/15/2020 Patch 8.3.7

Adding a little extra information here:

Aside from this achievement not being solo-able, something I learned while doing this with a friend is Zuraal tends to get bugged and when he is suppose to cast a player into the void, he can get stuck in a Fixate loop, where he will chase players with no one being in the void.

You can reset this easily enough by walking to the entrance of the instance or zoning out then back in, however, we found that as two DPS this was a constant problem for us where he would get stuck in the Fixate loop and never send us or he would send both to the void at the same time and reset himself. However, we found that having my friend (Druid) go into a tank spec seemed to help, but this isn't a confirmed fix to the issue, just a tid bit we experienced.

Conclusion:

This is do-able with a duo at max level (120), however, since I didn't see anyone experiencing this too much I wanted to make others aware that the "Fixate loop" was rather common. No specific fix that we could see for this, but maybe bringing a tank spec character along might make this a bit easier.