The boss events are on day 2 of any episode after you have completed all the initial missions for that day. You will see a mission to bang the gong. They always spawn in Kiichiro food court. When you get the gong mission just run to the location marked on your map and press B next to the gong to ring it. All players that are alive in the game must be near the gong to ring it. Once rang the boss will spawn. Just hack and slash him and you are good to go!
Bosses are not overly difficult in this game especially with 3 or 4 players.
The episode 3 boss can be difficult to kill due to a bug or a mechanic... not sure which it really is. The trick here is to not run from the boss. If he's running at full speed to catch someone he takes little to no damage. Use the dodge ability by pressing RB twice to roll away from his attack.
The 4th episode boss has a small mechanic to it. We always kill them in Acid, Cold, Fire order. Also the theory is if they get close to each other they will heal so you need to try and keep them spread apart. This fight is extremely difficult solo and still pretty hard duo if you don't have the right weapons which would be high damage range weapons when trying to complete solo or duo.