Soul Power

Can not solo unless you have pet. He won't cast Corrupt Soul unless someone besides tank is there. I tried with companion trinket but didn't work.

...and for those that don't get that, this achievement used to be named "I Feel Good"

Every 30s or so, he will target one of your party members and channel a spell that makes you spawn a Corrupted Soul Fragment, what you are supposed to do is to run as far as you can from him (20-30y), because when those souls spawn the start moving towards Bronjahm, and when they touch him, they heal him.

This is how my group did it.

We kited him around the platform that's right before you pull him, our DK froze the souls and our warrior slowed them with Piercing Howl. We carefully ran around the platform making the souls follow him.

While at it, we kept him around 40% hp. Once all 4 spawned, we started moving back to the center of the room where you pull him, and there we burned him down till he was dead.

Ez achiv.