Make sure to CC or otherwise stun one Restless Soul for at least one minute. Reportedly, the following abilities works quite well:
Druid's Entangling Roots
Hunter's Ranger's Net
Paladin's Repentance
Rogue's Blind
Abilities that cannot CC the Restless Soul:
Demon Hunter's Imprison
Mage's Ring of Frost or Polymorph
Monk's Paralysis
Rogue's Sap
Priest's Shackle Undead
Abilities that could help in case of emergency or at the very last second:
Death Knight's Death Grip
Mage's Frost Nova with Ice Ward
Paladin's Hammer of Justice
Rogue's Kidney Shot
After the Restless Soul is CC'd for one minute or more, it'll become an Frustrated Soul.
Kill the Frustrated Soul and then kill the boss.
https://youtube.com/watch?v=dEJ9JweMEZQ: