Is Bone Colossus boss bugged in multiplayer?

Probably nothing actually buggy here, but mentioning on pure "feel and eye test." ran this boss 2x in a couple area level 81 maps with a friend of mine in party, and not sure if it's a party bug or not, but this boss was practically untouchable! Died twice after handily completing many other maps/bosses prior. Only was able to grind the beast to maybe 85% health (his companion bit the dust pretty quick). Compared it to a solo run with similar map mods and both bosses practically vaporized. I know that content ramps based on the number of people in party, but this was noticeably different feeling than any other content we were running.

Anyways, just figured i'd mention in case there was something fishy going on there.