Board Thread:Questions and Answers/@comment-24269716-20140402220750/@comment-6697950-20140403051551

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.