3 Riptides firing 3 S8 AP2 Large Blasts at BS5 and denying cover only kills 2 warlocks? If that is so then either Riptides are just shit or the council is broken as all hell.
It's due to spacing. If he put them in a line you will only be able to hit three which is why you have to single shot instead of firing the template. He can still have a re rollable 4 plus invul.
With out using lights that's 4.5 hits, about 4 wounds, 1 dead. If you use marker lights you can kill 2. However that is not adding plasma or melta shots, or the possibility of double melta. If you add double melta that would be 5 extra shots since one would fail. About 4 would hit and that would be 1 wound again. If you use marker lights to ignore cover and to upgrade to bs than that's 4 dead. In reality you would be hard pressed to even kill 2.
This is why I ply daemons. I would like to see how my 20 man squad of daemonettes would fair against the council. If I get all the buffs I want than they will be +2 invulnerable, endurance, flame shield, invisibility. While on the council they will have enfeeble, misfortune, and hallucination.
Master 3 psykers vs Master 3 psykers would be interesting.