I'm a Soul Man

Managed to solo this achievement as a Blood Dk.
After taking the Construct, I put a prison on the blue portal for the first wave of souls. I used a stopwatch to track the souls timer, but I believe DBM can do it as well.
-T=0 s they get caught in the prison.
-T=27 s, second prison on the same spot (it will trigger just in time for the second wave to get caught at T=30 s)
-T=57 s, third prison.
-T= 1 min 27 sec, fourth prison.
Now you have 20 souls, you just need to prepare a big pool of fire and destroy the prisons (if you're a melee class, you can just wait), and the construct should be almost dead because of Socrethar casting on you. When the last prison is destroyed, all the 20 souls will run toward you and get caught in the fire, and they should all die at the same time.
Then you just have to kill the construct a second time and finish Socrethar.

There is no need to use the prisons or any other sort of trap while soloing this. In order to solo this, you just need to kill the construct get in it and walk in a big circle around the boss. If your circle is big enough the mobs will never catch up to you. When the fourth wave of haunting souls has spawned either wait for your construct to die or get out of it, then kill all the haunting souls with a mass AoE. Once you have done this kill the boss and you should have your achievement

As of 8.1 this appears to be bugged. After exiting the construct it remains at 1% HP and cannot be killed. This happened on solo attempts.