Currently, my guild is working on hardmode Twin Val'kyr in ToC 25 (we've beaten it several times in 10). We've tried many different strategies, but they all revolve around one thing: soakers.
If you're not sure what a soaker is, it's a person who is the opposite color aura of the group they are closest to, and their job is to soak incoming orbs before they hit the raid, as well as dodge their opposite color orbs so that they do hit the raid (and thus buff damage). A soaker has to be highly mobile, as standing still to cast is a bit of a danger to them, depending upon persistence of orbs.
But because the soaker's job is so vital, and because they are a little ways out of the group, when they mess up...it is very, very noticeable. And if one dies, it can often cause a chain reaction for other soakers to die, as they suddenly have to cover a wider range, with more chance of being swarmed by the wrong color orb. You can take a hit from one opposite color orb, but more than that and you're generally dead, if you're not a tank.
My guild spent 3 hours, 25-26 attempts, last night on this fight. I was a dark soaker for the light side the entire time. Sometimes I could execute it perfectly til a wipe; many other times I died early. Some of it is RNG; you have nowhere to go and too many orbs of the wrong color flying at you, and your Barkskin can't soak that much damage, or you get a debuff of the opposite aura at the wrong time. Some of it is a split decision gone awry; you make a pathing choice under pressure and it puts you into a dead end of your own design. Some of it is lag; with so many orbs flying around, spell effects and people packed into one space, your computer can hiccup you into a stack of death, or your latency can show the orbs to be farther away than they actually are. Some of it is just being human; it's a high pressure fight, you have a high pressure job (run, soak, dodge, heal, stay alive), and you're going to mess up.
The worst part is the feeling that you messed up, you know you messed up, and it's likely your mess up is going to cause a chain reaction of deaths, either from orbs getting through or because your other soaker(s) now have to cover more ground (making their jobs more stressful and have a larger margin of error). Besides that, your mess up is obvious, when a soaker dies, it's a Bad Thing. You can lose a DPS or two so long as your damage can keep up with shields and the enrage timer. When a soaker dies, it's often a multiple blow: you lost a shield for the raid and, in our strategy, likely a healer.
The only fight that has stressed me out more was trying to two-heal hardmode Anub'arak 10 with a Disc Priest, and not by a lot. Playing a role that is so crucial that, within the confines of the encounter's start to finish, you have a very low margin of error (and your errors in fact increase the others' margins of error), tacked onto a job that already had a low margin of error (raid healing for auras requires intense upkeep, especially on those with Touch of Light or Touch of Dark when there is no aura switching) is very taxing. And the knowledge that not only did you mess up, but some of the things you messed up to are out of your control while plenty of others were and you just made a stupid split second decision, and sometimes you're not even sure which ones are which...
Let's just say I feel rather terrible about last night and the wipes I contributed to. I feel even worse because I don't know if I am even improving at the job. I really hope I am, because we have 17 attempts left for tonight.