While Lu Bu wouldn't have invincibility during Musou, he doesn't really need it here: In his fury mode, he becomes fast as hell, and Lara and Vivi are just way the hell too squishy to be able to stand up to that, at all. The only problem is the distance. 50 feet is a distance, yes, but it's an entirely negligible one - that can be covered by an average human in a couple of seconds.
Video of Lu Bu in fury mode:
Not to mention that Lu Bu has pretty solid armor and rather super human durability, having stricken fear in the hearts of entire armies before. While a continuous onslaught from Lara and Vivi would kill him, 50 seconds is just closed too quickly, and his blades do too much to the thin, fleshy hides of Lara and Vivi for them to survive long enough for that.
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I'm too tired to drive anywhere, anyway, right now, do you care if I praise Black Turtle?
Call me numbers.