You Gotta Keep 'em Separated

Keeping the boss on the edge of the platform towards the green portal side to drop the fire patches to give the most time for them to spread was key to getting the achievement (and of course, requiring that no one messes up with dropping theirs). The imps died without trouble due to the melee keeping them interrupted or dead before a cast could even go off.

As for dealing with the shadow side, we dragged the boss all the way to the other side and did the same with the shadow pools as we did with the fire so they were kept as far away as possible. The void elemental adds don't drop any pools of their own if everyone dodges them correctly, making the remaining key being on the tanks. The tanks must be very fast to taunt swap in the phase after Omnus is killed when he changes colors to prevent the wrong strike attack from being done on them as the most likely cause of a Shadowfel Annihilation in this phase using the strategy for positioning comes from slow tank swaps.

I'm assuming this achievement gets it's name from the song "The Offspring - Come Out and Play (Keep 'em Separated)" since as soon as I read it that song played back to back in my head. x)

Easy peazy to solo- just burst damage and get him down fast. (ilvl 933 Unholy DK). Should be a non issue from now and after BfA drops.