So, after patch 2.28 we got this:
Boss enemies in certain FATEs will have their HP reduced proportional to the number of participants in the previous FATE.
Having to complete the "Breaking Dawn" FATE for my book, I waited about 30 minutes - 1 hour before it appeared, there were 4 other players with me, I was there when it appeared, no 20 seconds in the boss was already at about 1/4 health.
Trying my best with Energy Drain and Ruin II (both instant) I was only able to achieve Silver and of course that isn't enough for a checkmark.
After that, uncertain when it will appear again, I only knew it might take a while, sitting there for 3 hours, it appeared a second time but was much harder to take down, gladly there was another player with me, as it was very late on an EU server and you'd likely not get anyone to help out and fail it.

I do not complain about the wait times here but the minimum and maximum HP of rare FATE boss. Imo, they should be higher and lower respectively.