Complex sentence: Upon winning the boss fight, the player is awarded a score multiplier based on how quickly the boss was defeated, which is then applied to the score earned in the Nightopia section in order to produce the player's final score for that Dream.
Split into simpler sentences: Upon winning the boss fight, the player is awarded a score multiplier based on how quickly the boss was defeated. This multiplier is then applied to the score earned in the Nightopia section in order to produce the player's final score for that Dream.