The Nth Doctor: Via a magical mishap. Replacing the only girl with someone who looks nothing like her? Then do what Beetleborgs did, and say that a spell was placed on Jo which caused her to look and sound different, and then a second spell makes Muggles see her old self, but not the main characters (or viewer.) That's a smooth save right there.
Bad Ass: He delivers an especially Curb-Stomp Battle to the Beetle Borgs in their first confrontation, and could still defeat them (if not destroy their powers this time) in their Metallix forms. In fact, they were forced to rely on Roboborg whenever he would show up in person to force him to retreat until they got their Mega Sectra modes.
Villain Decay: Was basically unstoppable in his first appearances, before ending up failing to fight them again later; then again, the Beetle Borg got new upgraded armors, most likely didn't fall to the same tricks anymore, and he still could handle them pretty well in a fight.