Please, if you see that boss is left with a sliver of HP and your phantom is capable, get the f*** away from the boss and use your shield. The phantoms will surely appreciate you not dying stupidly in the last seconds of the fight.
Yet this is sorta way to troll people - find the ones who fight bosses for humanity and souls, die in the last seconds, get the boss drops and the souls from bloodstain later, leave phantoms with nothing.
I have noticed a lot of players Leeroy to take the last few hits of a boss fight (when the boss AI is most aggressive too) and get destroyed. Maybe they want the glory for the boss they didn't really kill. If I see this I usually drop a Prism Stone down on their corpse, bow to the boss or bust out some binoculars if they die.
Worst thing I ever did in response to a bad host was intentionally landing a finishing blow on Havel just after the host died. Guy begged me for weapons for 15 minutes and waved in my face as I checked the messages, I have no regrets.
It's not often I summon help for bosses but it normally goes like so: *summons the person* we look at each other and have a formal understanding of "lets go **** em up".
And we then proceed to do so. That I can remember I've only died on a boss with a phantom once...got cocky when it was my 1st time fighting S&O when we had just taken out smough and got impaled promptly after.