He's weak to slashes the same way Superman is weak to magic. It affects him like a regular person. It doesn't do more damage.
Luffy's rubber body, which makes him more or less immune to blunt force, renders him no less vulnerable to blades than he ever was. For Luffy, heat also apparently makes him extra squishy.
Weaknesses. Despite these advantages Luffy is not immortal. Though immune to lightning, he can be damaged by other elements such as fire or ice, attacks involving cutting or stabbing and Haki-infused attacks.
Luffy's rubber attributes are a result of him eating a devil fruit called the Gum-Gum Fruit Granting him a rubber-like body, his powers make him immune to electric attacks and most blunt forces, but he is susceptible to attacks made with a sharp object or weapon (eg. sword, spear).
However, he is still normally susceptible to sharp objects such as swords and knives as well as nonphysical attacks like heat, cold, dehydration, and poison (though after the timeskip, Luffy has been shown a strong immunity to poison due to his fight against Magellan in Impel Down).
Luffy can get cut by swords, as its indicated him being rubber and it being probably the only thing that can hurt him.
Due to his rubber body, Monkey D. Luffy (One Piece) is immune to blunt force attacks.
Kaido is one of the best fights that fans have seen in One Piece over the years, if not the best. The fight is, undoubtedly, the most difficult one that Luffy has fought so far. Kaido managed to defeat Luffy three times. For the first time, Luffy faced defeat against him Kuri and was thrown in prison.
However, much like many of his crewmates and allies, Luffy was once weak. His Devil Fruit made him even weaker as it was incredibly difficult for him to control his powers as a kid. Thanks to the relentless effort that he put in, he was able to grow up into a terrific pirate.
Luffy is a rubber man is because as a child he ate the Gomu Gomu no Mi (Gum-Gum Fruit). However, all users of such fruits share the same weak point: seawater. They cannot swim and lose all their strength when they come into contact with the ocean. This is one of the great ironies of the series.
Additionally, rubber has a maximum limit to how far it can stretch; in the first SBS, Oda stated that the user's maximum stretching distance is "72 Gomu Gomus". Attempting to go beyond this length can cause the user to bleed and be at risk of ripping.
Luffy Took 10 Years Off His Life by Altering His Own Hormones. Gear Second's life-shortening side effect is made worse by how often Luffy relies on the technique.
With his devil-fruit powers, Luffy is able to manipulate his rubbery body into various forms, which he calls “Gears.” Currently, Luffy has managed to unlock up to five gear forms, and his fifth gear is known to be his peak form.
21 Chapter 195 (p. 19-21) and Episode 119, Zoro first uses One Sword Style against Mr. 1 to defeat him by cutting through his steel body.
Luffy's gears also put pressure on his body. Since using them is obviously more dangerous than smoking or drinking, Using gears repeatedly will eventually shorten his life to say 30 or 40. His 2 year training probably made him resistant to immediate side effects like weakness and becoming a dwarf after gear 3rd.
Buggy is the captain of the Buggy Pirates and one of the Four Emperors of the Sea. A person who has not met Buggy can easily be fooled by his titles, but those who have know that he is one of the weakest characters in the series.
Monkey D. Luffy has developed an extremely high resistance (if not quasi-immunity) to most poison. Primarily due to his near-death experience against Magellan's poisonous Devil Fruit, he started producing antitoxins (antibodies) to combat future poisons and foreign toxins without noticing.
Dracule Mihawk is Zoro's biggest rival, being the man who holds the title of the World's Strongest Swordsman. Mihawk is extremely skilled with the blade and his swordplay is described as very aesthetic. He is also extremely good at using Haki given that he taught its usage to Zoro.
Having used his vast knowledge to create lasers, giant robots, and the Thousand Sunny's puzzle like architecutre, Franky has to have the most intelligent mind of all the Straw Hats.
Black's ruse also proved to be his greatest undoing since pretending to be Luffy made him a target of the real Luffy's enemies such as the marines. Sentomaru, having met the real Luffy before and was not at all convinced by the impostor's facade, confronted Black and put an end to his deception.
Throughout his journey Luffy has made strong bonds with numerous women. At least two have fallen in love with him, Alvida and Hancock, whilst another two have become enamoured with him, Rebecca and Shirahoshi. Nami, Robin and Vivi all share close and deep bonds with Luffy and have become some of his closest friends.
As it stands, it appears that he remains as a rubberman but loses his strength and becomes unable to control his power (for example, if his arm is stretched when he comes into contact with it, it flops around like a wet noodle until he contracts it) if he falls in water or touches seastone... But why?