The mob will deaggro and spawn adds near to 90%, 75%, 50%, 25% and 10% but not always exact.
When the Harbinger resets he will spawn 4 small mobs around him. Pull one of the quickly and dps it down quick. You need to kill it within 30 seconds.
After you have killed one, it will reaggro you and you do the same thing again but the next time it reset there will only be 3 mini mobs up, and so on and so forth.
The mob rarely resets a 4th time to kill the 4th mini mob unless you failed one too many times on previous resets.