From the Super Mario Wiki
A Dark Mechawful.5 (known as Dark Mechawful 1.5 in the EUR English text) is an enemy found in Mario & Luigi: Bowser's Inside Story. When Bowser destroys a Dark Mechawful and leaves the ejecting head alone for a few turns, the head becomes a Dark Mechawful.5, gaining a new metal body. The differences between the two are:
When Dark Mechawful.5 attacks, the numbers on its chest will malfunction when the timer reaches 2 seconds, making it much more difficult for Bowser to counter. Otherwise defeating it is similar to defeating the original Dark Mechawful, including the need to inhale the ever-persistent head to prevent more Dark Mechawful.5s from being generated.
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