How do you get the secret ending in Luigi’s Mansion 3?

How do you get the secret ending in Luigi’s Mansion 3? If you run out of health or fail to stop him in time, King Boo will again have a new painting to add to his collection. If you do beat him, you get your true ending. These alternate endings aren’t outrageously fancy, since there isn’t much decision-making involved.

If you run out of health or fail to stop him in time, King Boo will again have a new painting to add to his collection. If you do beat him, you get your true ending. These alternate endings aren’t outrageously fancy, since there isn’t much decision-making involved.
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