This MWC is random and can occur at any time of the day. People usually check the Adventurer's Guild board from 1 hour to 1 hour, example: 09:00, 10:00 etc. You don't need to talk to the NPC to "participate". When it triggers, the boss spawns according to the location of the bank that was robbed.
You can have 4 characters logged out on each boss spot and login to find it there without needing to talk to the NPC, for example.
If you find and kill the boss, you can deliver the bag to the bank that was robbed and get the respective achievements.