I'm not convinced Lu Bu can even tank a single shot from a high caliber weapon. It's something he's never come across. He lives in 400AD. He's utterly human and specializes in hand to hand combat. Unfortunately, both Lara and Vivi specialize in long range.
It's like bringing a knife to a gunfight.
Lara could sink him with a head shot. No way Lu Bu could survive a shot to the face or any other part of his body. His armor was made to deflect iron spears. And if all else fails, Vivi could roast him with a meteor. This fight is as one sided as a fight can get.
It'd be like watching the end of Last Samurai, when all the best samurais in the land get mowed down by two or three gatlin guns.
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All you need is love.