This is possible to do solo and I will share how I did it. I would still recommend doing this with other people.
Character - Claire level 50 with level 5 weapon and armor
Combat Art Light - Judgment with [Inquisition] and [Vengeful Trial]
Combat Art Heavy - Divine Aura with [Whirl of Punishment] and [Exalted Soul]
Defensive Move - Dodge (for mobility)
Weapon and Spirit choice isn't important.
I highly recommend doing this on Deity difficulty as I did, I could NOT get this to work on lower difficulties as I kept killing the boss by accident. Playing on Deity also allows you to retry by dying if you fail the requirements. For those curious, this took me roughly 15 attempts once I worked out a strategy.
This is done on the level Zhurag on the boss The Unspeakable Evil.
**Make sure you have full energy before engaging the boss**
The strategy is to apply [Judgment] to all the orbs and have any damage trigger it (this removes the shield and deals damage). [Divine Aura] is used because it has a huge radius and was able to trigger [Judgment] and kill the orbs in one hit.
Position yourself and the boss in the middle of the room, and bring his health down to roughly 70% so that he spawns the orbs.
Immediately after spawning the orbs, activate [Divine Aura] and apply [Judgment] to all visible orbs in all directions (you should be able to reach most of them from the middle).
Once you think you have applied [Judgment] to everything, roll around the room to kill the orbs.
The achievement will unlock as soon as the boss returns after the orb phase, if you didn't get it you need to die and try again.
I have tried to make this as clear as possible, good luck!