During the Summit War, Whitebeard used his power to decimated Akainu during their fight, in fact one of his quakes was able to harm Akainu even when parts of his body were made of magma.
Akainu's weaknesses are drowning and caught off guard by Seastone.
Can he beat Akainu? Yes, he can, as he is a Yonko tier being now. Prior to his awakening, Luffy was able to blitz Kaido to the point that he needed Future Sight to keep up.
Yes,at the time at Marineford, Garp could have killed Akainu.
Whitebeard, on the other hand, easily equaled Roger's power. Even Garp called Whitebeard "The King of Seas." So, it is beyond any shadow of a doubt that Whitebeard could have defeated Garp in a fight to the death.
Although he is only a vice-admiral, Garp is the strongest Marine in the series, surpassing even the likes of Sengoku and Akainu.
1) Kaido. Kaido did in fact lose to Luffy's Gear 5 at the end of the Wano Country arc. However, keep in mind that he withstood multiple waves of strong enemies before this final encounter. Luffy was also completely worn out after the fight and fell into a coma for about a week.
Zoro has been constantly improving at a rapid pace in order to fight the monsters of the New World. He has a lot of potential to still improve and become one of the best, however, there is no way that he is beating Akainu.
Luffy's Gear 5 makes him incredibly powerful, more than most Naruto characters. In fact, it's arguable he could defeat all of them, bar the absolute strongest ones.
Shanks and Akainu have clashed in the past, but Akainu was only an Admiral, and he was aware that he stood no chance against Shanks. Needless to say, Shanks is still comfortably stronger than him.
Akainu has far better feats and scalings.
In terms of Haki, Akainu is certainly quite impressive. He has already been confirmed to be a user of Observation and Armament Haki. During the Marineford arc, Akainu could survive some mighty powerful blows from Whitebeard thanks to his incredibly tough Haki.
During the Summit War, Marco clashed against all three of the Admirals on an equal footing. He managed to hurt two of the three, showing that he's indeed capable of fighting and possibly even beating them.
Luffy is close to Shanks in terms of power and it is only after their fight that he will officially be able to call himself stronger than Shanks.
Can he beat Akainu? Yes, he can, as he is a Yonko tier being now. Prior to his awakening, Luffy was able to blitz Kaido to the point that he needed Future Sight to keep up.
Garp Is Considered To Be One Of The Strongest Marines. Also renowned as 'Garp The Hero' and 'Garp The Fist', Monkey D. Garp is a Vice-Admiral of the Marines. Being their strongest known member in history, there's no denying that Garp is capable of taking on Kaido.
So yes,Luffy can take the Admirals as well as Smoker. I think that we can only wait and see how he does against a Logia user, as he is soon to fight Ceasar Clown.
Though he is a Marine and is loyal to the World Government, Garp is disgusted by the World Nobles and refused the promotion to admiral due to the fact it would make him their direct subordinate.
Gear 10th Luffy is Luffy's ultimate form from the upcoming speculative fan-made sequel to One Piece, Son of the Pirate King. Luffy has continuously trained his haki and Devil Fruit powers during his time as Pirate King until he achieved this gear.
Mihawk is said to be close to the same level as Shanks and is even dubbed as the greatest swordsman, so to say Luffy would defeat Mihawk is still debatable in the eyes of many fans.
The concept of power up Gears has been a core part of the One Piece franchise and allowed Luffy to take on the most difficult of foes. Fans speculate Gear 6 is inevitable – and with the series winding up to its conclusion, it could play a pivotal part in the final conflict.
Before he can finish, he is shot to death by Pooh Percy (now an Ellen Jamesian) inside the Steering gymnasium in front of Helen and his wrestling team. An epilogue summarizes what happens in the lives of the other characters after Garp's death.
Currently, Ryokugyu, Fujitora, and Kizaru are the three Admirals of the Marines in One Piece.