I'm fairly new to Blood spec. I solo'd Naxx 10 including Raz in blood spec with my DPS gear without much issue. I think Raz gave me the most trouble, but at the time I didn't quite understand the mechanic behind Blood Mastery's damage shield well enough and I was taking some serious damage I could have avoided.
I've since tried Raz on 25-man twice, and didn't even drop him to half health as at least one of his attacks was pounding me for around 70k.
My strat, or lack thereof, was to simply focus on Raz and use the adds to build up Vengeance, popping army at the start to give me a little distraction while so many adds were alive. The adds die pretty quick just taking the cleave and disease damage.
Any suggestions? Should I focus adds down first? My tank gear is terribad (vendor 408 greens and a smattering of 4-something higher greens from quests in Town Long Steppes.
Edit: Oh, and the gargoyles in plague quarter on 25-man can die in a fire. I have a devil of a time killing them without them stoneskinning.