Weird. I'm not sure what you guys are doing wrong, but I've never had a single boss on my list or my friends list (that I know of) that didn't get defeated.
Also, I have a 1-girl "team" that I save in cases of Seed Bearers or low hp bosses... to help entire friends lists get quests/drops from that boss. I do this for the benefit of OTHER people (it doesn't benefit myself in any way), but apparently -I- am the asshole. (pro tip: the people who 1-shot allies' Seed Bearers are the actual assholes). I admit I have accidentally used this "team" on 10m+ health bosses on occasion, but those bosses have always gone down within a few minutes (sometimes seconds) afterwards anyways. Way too fast for the rest of their friends to get in on the action. I have literally never had a problem with a teammate's boss not being defeated, even with 50+ active bosses on the list.