I was going to suggest doing the shield earlier, if low raid aoe dps is an issue
No joke, I solo'd 12 of the Spirits as Affliction on our heroic kill. SB:SS + Glyph of Soul Consumption means I go in alone for the last totem before burning through the last 25% of the boss. Pop Dark Soul+Racial, SB:SSx4, Drain Soul, repeat until you have to leave, win game.
Our MT (Null tank) Found that he can shield the first 2 shields. First shield he can do straight up. Second shield we just stun/knock back the adds and he can null the shield at the very end. Making it easier to dps the boss down.
For the Spirit Realm, I highly recommend Destro with MF. You can go into the spirit realm alone, cleave the entire room to death within 4 casts, heal to full, and jump back on the boss with a nice buff and full embers. In the 10 man game, I'm generally over the 150k mark. That leaves our players to grab a buff and jump out, or save the debuff in case we get dolled in a bad order.
Pick up MF. As soon as you see your screen change going into the Spirit Realm, put down a RoF, anywhere really, you're just waiting for adds to show up. Doesn't always happen, but sometimes it takes a second to fully get into the other realm. Pick a target that's pretty close to the middle, use Fire and Brimstone, Incin, Fire and Brimstone, Conflag, Fire and Brimstone, Incin while keeping up RoF in heavier populated locations and before you leave, snap off some Shadowburns to ember refund and bank back up to 4. You'll generally have time for 10 well planned casts in there before you have to start worrying about time. A few repetitions down there and you'll be surprised just how easy it is. At my gear level, I generally 2 shot all of the small adds and 4 or 5 down the Severer of Souls. The Severer's generally spawn in a decent spot to AoE anyway, so casting on them initially and spreading from there seems to work for the most part. Few times down there and you'll find yourself with nothing to do and everything dead . I've yet to be able to replicate that as Demo or Aff.
Although destro is fun for the spirit realm, it's not necessary IMO. Aff has overall more dps, so if dps on the boss is an issue with enrage, you're better off going aff. Also, affliction does reall well in the spirit realm as well. Agony hits really hard on the adds, especially once you get your spirtual innervation stacks up. You can throw out a bunch of SB:SS on the adds, or just manually put up agony and optionally corruption on them before going down. Agony lasts a long time and our dots keep ticking after you come back, so they'll eventually kill the adds.
Glyph of soul consumption is a must for any spec. Make sure to use it to heal yourself up with healthstone, mortal coil, and drain soul execute on some adds to get spiritual innervation to help rape the adds.
I do far more boss damage as Destro simply because I don't need to spend nearly as much time in the spirit realm as I do as aff.