LK heroic to 41%. Not there yet, but it's a start. It seems to be like the third fight or so that actually challenges me, though on the flipside I don't find it particularly hard - rather 'unforgiving' if anything. You don't immediately heal up an infest, people start dying. You don't immediately dispel the shambling horrors, people start dying. Two-shots on the tank are back too, and depending on how you get into the second phase one healer getting picked up can easily mean that either the first Infest kills DPS or that the tank gets 2-shot when Soul Reaper goes up.
What I do now (typing this out more for my own use than for you, I guess);
Start: At ~26 seconds to go on the RP, ProM on the MT. At 22 seconds, PW:S the entire raid. Back up into my own group, drink up, cast Devouring Plague on the LK.
P1: Smite X5 for Evangelism, then re-shield everything (the first Infest will have hit) and use Archangel once the shields are up. Also throw down a ProM if there's time. Additionally, while that's going on, dispel whoever gets the disease in time and keep an eye out on the tanks - if my partner gets the disease and needs to run, the off-tank is gonna drop to 50% before he can cast more heals, and an off-tank on 50% means he's likely to get 1-shot by an enraged horror. Near the end of the phase, move to the side way early so I can cover the tank healer while he's running. If you get an infest while moving to the edge without having everyone pre-shielded, you're boned.
Aside from all this, be aware where the Shadow Traps are going down so I don't run into one and be ready to use Arcane Torrent to fill in additional mana when Archangel isn't cutting it.
Phase transition: PW:B the melee and tank to lower their incoming damage. Whenever there's time, Smite X5 into Archangel. Otherwise keep an eye on the off-tank, dispel diseases from him as needed, throw up a possible PoH and if I didn't perfect P1 also cast Hymn of Hope to get mana back. Pre-shield everyone near the end of the phase or P2's first Infest is gonna cause havoc.
P2: Pray everything in the phase transition went well. Move in for the Valkyrs while at the same time keeping some instants up on the tanks, heal up the people once Infest hits (2-3 shields will have been taken away by damage in the transition), hope you don't get grabbed by the Valkyr, move out for Defile, somehow cast five Smites for more Archangelism. If the first Valkyr and Defile are down stuff and everybody is still alive, stuff stabilizes a bit and I can go back to the 'rotation' I had in P1 while still moving in/out for the Valkyrs/Defile.
And that's as far as we've gotten, though P3 (whenever we're not in frostmourne) should be just like P2, except a little bit easier because there's no Defile anymore. It definitely is one hell of a fight, though we should be able to get it down if we stop wiping on silly stuff like the tanks getting 2-shot. With only 1 or 2 more raids to go, pressure's mounting =(