Aye, Sarshan is pretty brutal with his goo attack. I've found even a double save or die (i.e. needing to fail two saves) is still fairly harsh in 4th Ed. After all, I've watched 5 consecutive failed death saves (obviously killing one PC while another almost went down).
I think the one thing a Paragon tier party should probably look at is powers that grant allies saving throws -- particularly ones that give a bonus to said save. Save Ends effects are not that common (or powerful) in Heroic Tier, but they definitely ramp up in Paragon. Simply giving your allies a bonus on their saves, or an extra save, etc. might have been enough to keep them in the fight.
Also, you may want to take a look at Aura Killing weapons. Sure it uses up an item daily power, but it can be huge -- heck, you don't even need it on your main weapon or have a high level version of it.