Generally you’d probably just want to use something like a REST API connected to a database that houses all of the boss information that can be modified, then when players initiate their version of the fight they make a request for the current boss HP, then that is set and their fight begins. The Dragon’s Dogma fight of the same type had a timer in which you’d do the damage you could and at 8 minutes the dragon would fly off and your damage was sent to the database to be recorded. Depending how worried you about cheating, you may need to have some countermeasures, like validating the fight, score, etc before accepting the score. Alternatively if you need more control, you’d actually have the player in a networked environment solo and control the fight with server authority there so the client couldn’t attempt to spoof scores easily, but since it’s singleplayer that’s mostly futile.