The last boss in SP should be easy if you have poison cleansing totem bound to a key on your main char (or an alt char but with a stopcasting on that char before placing it.), and whenever he does the aoe poison, immidiatly drop the totem (stop your current cast for it.) All the while being close to each other, yet in a bit of a cirlce.

Just a tip, I've not tried the end boss as a multibox on heroic so can't judge that well, but I have done it plenty as a shaman solo playing. And dropping poison cleansing straight up really saves the day. (As well as taking the druids NR buff and putting down the NR totem.)