What would be nice to know is exactly why you are wiping. Is it off tank death? There is an enrage for the Shamblers that needs to be tranq'd. That will solve that problem.
Is it infest? That is an ability that does 7k dmg to everyone and keeps ticking until the target is over 90% health. The longer it ticks, the harder it is to get rid of. Disc priests work great for this, but with the buff, it usually isnt that hard to deal with. Its on a strict timer, so you can pre-hot, shield people and even get them to use healthstones, etc. This is a two second cast, so there is time to get heals going.
Is it main tank death (not likely if you are on pace to push before the third Shambler)? If so, it could be that LK is getting too many stacks of Plague Siphon. Each one give the LK 2% more damage and he gets one each time Necrotic Plague jumps (times out or is dispelled). If youre bouncing this around amongst raid members, that could be bad.
The little adds hit for immaterial amounts. Even when a healer grabs one for a bit, its not significant damage (just annoying). I doubt they are your problem.
My gut feeling is that you are wiping to one of the three mechanics above. Let us know which one it is and we can help you out. Even better, if you have combat logs of wipes, there are guys on this site that blow me away with the statistical analysis. They will fix you up immediately.
Deeps for show..... tank for dough.....