The key mechanic for this encounter on both normal and HC is Saurfang's blood power. We can easily brute force normal with our dps, making it trivial. However, on HC he gains blood power much much quicker. It may help for everyone to refresh themselves as to the sources of this:
Melee swings (hitting the tanks - avoidance is goooood for this fight)
Rune of Blood (tanks only - fast tank swaps are essential)
Blood Nova - people standing too close
Blood Boil - dot placed on random target
Blood Beasts chomping on people - 'Blood Link' iirc
Feel free to add any I've missed (durp durp) and/or call me a nub! Anyway, each ability feeds him blood power much much faster on hc. Our problem is that he's gaining blood power (and therefore Marks of the Fallen Champ) too fast. As Darieth said last night, it's a soft enrage mechanic. So, how do we beat him? We either need to reduce the speed at which he gains or get him down faster (both would be just lovely!), which will reduce the number of Marks he gains overall. This is tricky to balance. More tank healers will not help this. More raid healers will help the healing but hurt the dps. In short: taking from Peter to give to Paul. It won't work. Moar heals, moar pewpew and definitely less blood power will help. Blood lust on pull next time we try it will give Darieth and Akee a nice little insight as to if and how quickening the kill could work for us.
P.S. I'm reliably informed by Darieth that shields, while helping with the damage, won't reduce the amount of blood power gained from Blood Boil on his random targets for this. Immunities from damage do reduce the blood power gain though. So, everyone needs to do some homework: figure out a) whether your class offers an ability that can help in this regard; and b) learn to use it under the pressure of this fight.