Ok, I made a picture to show it easier.
So, on phase 2 (with valkyrs) warlocks can stay in these spots.
When they have defile, its next + for raid because they can easy avoid it and melee with tank don't need to worry about it.
When Valkyrs spawns they catch random 3 players (or 1 on 10man) and fly to NEAREST EDGE. So if valkyr catch lock, raid can save much dps and just ignore one of three valkyrs, when valkyr drop warlock, he use teleport and he is save.
Its really make whole fight a lot easier.