Your shadow priest is not even attacking the boss. He's LITERALLY spending the entire fight bursting down spears. You guys are losing like 10 million boss damage from him alone.
Shadow should be pretty much at the top of the 'useful' damage for this fight and especially in the last phase when boss dps is so important he should be shining. He should be maintaining swp ONLY on the spears for most of the fight, and pumping all of his mind blasts and devouring plagues and literally every other spell into the boss. Obviously you have to kill the spears when the encounter calls for it. But there is absolutely no reason for him to be killing spears in the early phases. And there's no reason for him to NOT be maintaining 50-60k+ single target damage on the boss in the last phase. Instead, when you guys hero, he spends like 30 seconds doing half-assed boss damage, and then goes back to doing ??
Surge of darkness is also probably the worst talent choice here. You don't really have enough free globals to waste on mind spikes. Insanity and mindbender are both fine.
EDIT: On some pulls you have 2 shadows. They're both doing the same thing. I am confuse.