In Boss Raids, everyone in your Messenger chat group is fighting against one really tough Boss. The chat group will have a limited amount of time to do as much damage as you can to the Boss! When the Boss is defeated, everyone in the chat group that participated receives rewards. The Boss must be completely defeated for everyone to receive their rewards.
Each Messenger chat group contains a separate Boss Raid, so you will be fighting different bosses in different chat groups. Rewards received must also be claimed in the same chat group.
The Boss Raids reset back to level 1 every week on Monday, 11 AM Pacific Time (as indicated by the countdown timer).