You can create an event that changes the position, to do so:
Go to View -> Events -> Add -> Change the screen to how you want it to be at the fight -> In the section "Position", click "Use Current" -> Change the screen back to how it was before the boss.
If you want the screen to change back to normal after the boss fight, you will have to create a new event that is really similar to the one that starts the boss, with the difference that this time the screen will change to a wider one.
Also, don't forget that something has to trigger this event (to achieve something similar to Kirby's midbosses, axes and/or an activate event can be useful).
This video might help:
https://www.youtube.com/watch?v=ZImJSVw0ndk&t=711s 
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