I don't know what it's like in the real 4 player dungeons, or open world boss mob kills, but playing through Crow's Wood with a party only 1 person got credit towards the achievement for killing the boss mobs in there every time we killed one, and it seemed to be whoever hit it first. So we had to wait for spawns and cycle through a few times. This should never be the case - all members of the party should be given credit.