I see a lot of Defilers having this problem in grps I am in with my coercer and I don't really understand it. I spent a lot of time raiding with my defiler a couple of years ago and have occassionaly blown the dust off my defiler by request from friends. I still don't have this problem and my advice (which most defilers are offended by and I don't know why) is to preward with your grp ward and after the pull use your single ward. I do cancel my single ward before each pull because it will cause aggro. I use profit UI and when you get used to it..it will be a habit...it will also keep you alive
Because Defilers are so good at generating power I focus a lot on Stamina and have my health up to 10,000 so in the event I get the first hit I can take it until the tank pulls it off me.
As a side note...my defiler is no longer in "raid gear" and imo would be considered average with T2 shard armor and adpet IIIs
Hope this helps some.