If I may make a suggestion about boss fights, This is how it worked in a Sword Art Online-based RP I was in (where boss battles are central to the RP), with about 5-6 players, and this worked out just great.

Each participant in the boss fight decides their own character's actions and the boss's reactions, including the severity of the damage inflicted. The GM or chosen RPer decides when the boss is finally killed, and concludes the boss battle in a post that wraps up everything done by all characters, including their own. 

This removes the need for a player dedicated to controlling the boss and the hassle involved with responding to every character's actions as they are posted. Just one player (or the GM) would be needed to summarize player actions during each posting cycle until the battle ends.