User blog comment:AYET/Canon death for Luffy/@comment-4889755-20140104045041

His plot armor is his biggest advantage.

Plus his immunity to any poison, electric, and blunt force attacks. Plus that haki.

So if he were to die, it would have to be an extreme veteran pirate/marine (think Whitebeard/Garp) who uses swords/sharp pointed objects. Plus this person would need all 3 haki.