/cast [nochanneling] Mind Flay


is what ur looking for. that condition prevents you from casting anything in that macro if you are already channelling a spell..

so urs would be

/assist Main
/castsequence reset=24/combat/target [nochanneling] Shadow Word: Pain, Vampiric Touch, Vampiric Embrace, Mind Blast, Mind Flay, Shadow Word: Death, Mind Flay, Mind Flay, Mind Flay

i think