I only had this to work 1½ times in a total of 500+ tries.
The point is to solo any given boss without moving, which makes it completly bottable.
What i did:
1. Pull boss.
2. Wait for it to move near you/for it to target/start running.
3. Feign Death, now the boss will either run back to its pul point/stop attacking, or dissappear.
4. Now asap if timed correctly, you will be able to dps the boss without it moving(this is how i killed it).
OR
5. The boss will despawn, but when it respawns, it will have lost the amount of dmg you caused(this happened to me once, but then i failed at around 29% hp which caused the boss to respawn with 100% hp)
6. If you manage to time everything correctly, you will be able to kill any boss
Enjoy.
ps. I'll try to do this again if possible and screenshot it.