I think you can ever kill some 25man bosses in dps spec (everything is doable in dps spec in 10HM, except for Saurfang and Lich King. Well, I never tried Lich King in dps spec, but I don't think it's possible.)
But yeah, if you can't survive in dps spec, use dps gear in prot spec. And make sure you have 7.5% Hit/15% Expertise, or as close as possible depending on your gear. With a low ilvl gear, I recommend the Ghost Iron Dragonling and the dps trinket from Scholomance.
As I said above, I never tried ICC 25 (except for Marrowgar), but I know some people killed Deathwhipser 25HM, so 25NM is definitely doable.
Edit: Damn, you said "Deathbringer", not "Deathwhisper". Sorry, not used to english names of these encounters. :p
Well, I don't know if his enrage timer is doable in 25NM (and I'm pretty sure it is not doable in 25HM currently.) Be sure to get your Hit/Exp cap and try to maximize the uptime of SBar, he won't heal if you absorb his attacks. Use your defensive Demoralizing Shout/Demoralizing Banner/Shield Wall as often as possible, the less it deals damage the less he will heal.
And, last but not the least, if you can manage to get 2p T13 (yes, T13, not T14) it will help you a lot. Each Revenge triggers an absorb, so Saurfang will heal less.
Edited by Grothendieck on 15/01/2013 01:09 GMT