I use boomkin shrooms and our mage does the explosion and that's pretty much it. I mark the tank with a star so it's easy to see him when he moves and as the RL I call out when to stop moving and collapse, typically 5 seconds left. I place mushrooms while people are closing in and we're all out instantly. Be warned though, you can get the shadow orbs at the same time as a shout which slaughtered our group once. We just called it bad luck and did it again though.
If people are getting gimped in this phase it is likely that someone is using a large single target spell (i.e. Chaos Bolt) when all they should be doing is some gentle AoE.
If you are still struggling just make sure the raid is topped before you stack for the shout (use a CD if you like, you barely need them elsewhere). I use Spirit Link when I can, it's Godly for this mechanic