Then there are other games that drop you into a Boss Battle right from the very beginning.
While not always, this boss will more than likely be the game’s Big Bad, and usually also the Final Boss. If it can be beaten, either then the boss will be in its weakest form, or the player is in the A Taste of Power segment and soon will be depowered. Otherwise, it will be a Hopeless Boss Fight. If it’s not the Big Bad, then it will likely be a Starter Villain.
Starting in medias res is really common, actually. Not just in games, I’ve seen plenty of media where it opens with a confrontation with the Big Bad who is going to be the Final Boss.