I think that Natsu can take out the rubber man because he can manipulate fire, which in case you don't know burns rubber, and while it's not stated in One Piece that Luffy's weakness is fire, Luffy has shown significant difficulty against fire based foes and he has even said that he could never defeat his brother, who can also control fire. While Luffy has Haki, Natsu has fought people with the same abilities as Luffy like the poison dragon slayer, and Natsu actually won that fight so I think he will have experience against opponents with precognition. It has been said by Erza that Natsu's skill and fighting power depends on the opponent he's fighting and Rogue said his power is limitless, so I don't see Luffy overpowering Natsu. Luffy's Red Hawk ability would rove useless against Natsu and even if Luffy turns his skin to steel, Natsu has been shown to be able to break steel in the past. Luffy may be fast, but Natsu has been shown to have the same kind of speed as he too is able to dodge incoming attacks like when he fought Sting and Rogue. Unlike Luffy, Natsu does not get tired so easily during fight as shown with his fight with Sting and Rogue and he is just as durable to as he was able to survive being hit through several layers of the ground and let's not forget Natsu's enhanced senses which would counter Luffy's supersonic speed ability as Natsu could smell him out. While some may argue that Luffy's moves are unpredictable, Usopp was able to predict Luffy's moves, and he was actually able to create weapons designed to take out Luffy. Natsu was able to predict Sting's moves during their fight so I would argue we could do the same with Luffy. Luffy may be extremely strong, but don't forget, in the Fairy Tail universe dragon slayers have the same powers as dragons and since it is implied that dragons are extremely strong, and it is also said that Natsu has not harnessed his true potential, I would say Natsu may be stronger than Natsu. It is not true that Natsu needs to recharge by eating fire as shown with his fight with Sting and Rogue, even without fire, Natsu became even stronger in the final fight with Sting and Rogue. Furthermore, Luffy is not a better fighter than Natsu. Luffy is not a trained masrtial artist, it just seems that way because of his agility and without his devil fruit power, Luffy has no real fighting experience, and while his boxing match with Foxy was impressive, Natsu has been fighting his whole life, even as a child, and this experience would give Natsu a massive advantage over Luffy. Now if Natsu was given Etherion and Jellal's fire, Natsu would be even more unstoppable as Natsu's speed matches Luffys with Etherion as when Wendy was exposed to Etherion, she was able to move at super sonic speeds too. Luffy's Gear Four may be the biggest threat to Natsu, but keep in mind, Natsu was one of the only people to actually defeat a dragon, and Luffy's power is nothing compaired to them. Therefore, Natsu could beat Luffy. Now prove me wrong and tell me why Luffy would win.