Supermassive

I was so excited that I figured out a way to do this that I just had to make an account and post it for everyone else. Using my warlock, this method was super easy!

1) Summon a Voidwalker, turn off his Consuming Shadows and set him to passive
2) Start the encounter, then use the Voidwalker's Suffering Taunt, then set him back to passive so that he doesn't continue to attack Algalon
3) Use the Move To ability to have the Voidwalker tank Algalon off to the side so that he's out of the way. The Voidwalker will keep taking Punch Face hits, but wont get put into the black hole. Every so often Algalon will make more Collapsing Stars. You can stack as many as you want. Since they spawn in the same locations, the worst case scenario is that you'll need to wait for three rounds of Collapsing Stars.
4) Set up Demonic Circle on one side of the black holes and then hang out on the other side.
5) Wait for the Living Constellations to descend on you then use Demonic Circle: Teleport to make them come through the black holes!

Worked like a charm!

Solo'd this as a balance druid. RNG is involved, but is easily reset by entering a black hole if needed. Took 5 tries in total.

-Kill Collapsing Stars as close as possible to each other, not at the edges of the room
-Drag Living Constellations over them as quick as possible
-Once you do this part of the achievement, all you have to do is kill him. Even if he is reset, the previous closed Black Holes still count towards achievement.

Sill soloable in 8.2.5

Recommended character use: Brewmaster monk

1. Summon Black Ox Statue to aggro the boss (try placing it near the edge, you may need to place it a couple of times).
2. Kill Collapsing Stars close so it turns into black holes (Do not step on it as it will lead to a reset), you may need for 3-4 rounds to do so. Kill the Living Constellations as it is useless at this moment.
3. After step 3 completed, kite the Living Constellations into the 3 Black Holes within 10 seconds
4. Kill boss
5. Achievement!

Good luck~