So Floor Bosses are the strongest they require a whole raid group and smart tactics. 4 life bars(up to Floor 75). What about Field Bosses? Does a Raid Grou requred or a single party defeat one? 4 life bars as far as I know(probably up to Floor 75). What about quest bosses? Since there are solo players quest bosses should be soloable without mass overleveing. How many life bars do they have?
A raid group is only required for floor bosses. Field bosses can be handled with a party or two, depending on how strong the members of the party are. I don't remember whether it was said how many HP bars do Field Bosses have, but there haven't been that many Field Bosses described in the story.
As for quest bosses, it really depends on the quest/event. Nicholas (with 4 health bars) was beaten by Kirito alone, but with an overly large amount of healing crystals and even then he was barely alive in the end. On the other hand, the lungfish was a piece of cake.