Ever since I started to pull him into the bone part (park DPS/Healer on the top of the bones, and tank at bottom of it) I have not had any trouble at all with Slad anymore.

Edit: For Moorabi: Now that I no longer have a problem with this guy and my gear is still the same as before this is what I do. 4X Healing Stream totems and attempt to keep him in Troll form. If you can't its still easy just go all out DPS on him (Save Bloodlust/Heroism and Elemental Mastery for this phase). Make sure your tank has full health for this phase no matter what. Once you see his cast bar for quake stop DPS and spam heal LHW with all 4 shammies to your tank to keep him up. Once you get him full just spam a chain heal and you should be good, and continue with the all out DPS. I don't ever get hit by more then one quake now while he is in Rhino form anymore.