I didn't find health much of an issue, but the DPS gain you get from the echo was far too low for very small groups.

With 4 people in Castrum, we could not defeat both bosses before the Tunnel Armor fell.
Managed to 2-man the first boss in Delibrum Reginae, which took 20 minutes. We decided to stop because we doubted we could beat the final DPS check on the Queen's Guard.
With 4 people, we ran out of time while fighting Cuchulainn. We made the mistake of sending 2 people to the top at the first part of the dungeon when we should of only sent 1, but it was still very slow. I'm kind of curious if we could even continue after that since you need 4 people to stand on the button to open the doors on the sides.

Best thing I can suggest is to put up a Party Finder and hope you can find 8 people that need it.