And also, you'd probably be better off to use Shadowmeld as an extra stun for the sons on Ragnaros.
Ok, it'd be ready in P4 then, but the constant damage you get there while kiting will render Shadowmeld completely useless anyway.
Indeed, an extra stun in the sons really help out a lot once in a while so you get all the sons stacked or save a wipe. Tho, don't forget about diminishing returns because the adds will get immune for stuns after the second one, regardless of rogue.
Anyway, here's the macro I'm using in case anyone needs. I suggest using the macro not instantly as the nameplates spawn, but a few after it untill the whole summon animation ends (sun starts growing big from ragnaros elemental seed).
/target Son of Flame
/cast Cheap Shot
/s >> Stunning this Add <<
EDIT: You can use /targetenemy (it will target the closest enemy) as another solution to /target Son of Flame tho I never had an issue with it.
As assassination, on both Beth and Domo, it is possible to rupture the adds (Spinners/Cats) and gain a higher energy return when they die, allowing you to get another mutilate (and often rupture) up on another add/the boss. This can be chained for quite a few adds.
Indeed, this works out really fine.
Also, if someone's progressing in Heroic Ragnaros as Combat and is getting energy capped for GCD/Feinting the first Trap due to haste trinket procc's, Cloak of Shadows workss out fine due to being of GCD. Still preventing the damage from the trap and enabeling you to not cap that energy due to Feint GCD.