I have absolutely no idea, but that's fascinating... have you tested in other areas of the game if Skullcrusher always seems to take the same amount of damage like that on different hits, too? 

No, I haven't. I may check it later. He is not in my usual dungeon teams.

This was my first attempt to gather information about boss stats, both ordinary and unique like how much damage Gathering Fury adds. I know his Speed, Resist (found a post with it) and Defense (calculated it).

I recorded a video, then wrote down all the numbers.

The ultimate goal is to develop clan boss simulator that gives expected damage with given champion stats and skills. Chance of that happening is less than getting a legendary from a void shard, but it's the path that matters, not the final result :)

I tried him in Dragon. He received 3 different damage values from Tayrel AoE, the same values from other champion's AoE, higher amount from single target hit and quite high damage from Dragon AoE. In all cases damage he received was of the same magnitude as protected champions. Values were 570, 600 and 660. Interesting that the range is wider than 589-660 observed in Clan Boss.

So the "3 values" part happened again, but there was no abnormally low damage.