I mean, the issue is simple. People's boss dps is ass. If you're that high ilvl and getting a 3rd wave of adds then your dps just need to learn to stop padding on adds that don't need to die instantly as we for example don't get the 2nd warsong so they don't actually melee for any damage.
Other than that, should either solo tank or solo heal the fight, it makes the dps check much easier, and helps a lot with the transitions.
Speaking of transitions find a way with your comp to SAFELY engauge all 5 packs of adds, for example the back two are engaged by me on a WW, and our ele shaman and we both just stun lock our respective sides and put as much damage into them until the front groups get up there. This makes the transition much smoother as long as you don't let casts off still.
http://www.warcraftlogs.com/reports/...done&target=59
There's our damage breakdown in the 1st transition from our original kill. Splitting dps around all the packs allows everyone to actually contribute dps instead of nuking a pack down really fast, walking to the next pack and having it take longer because they're not already down to half health from someone being there already.
Tl;DR Push your dips.