Yes,at the time at Marineford, Garp could have killed Akainu. We have seen the power level of Akainu during his fight with Whitebeard so we can safely assume that he is greatly dependent on his DF power & some degree of mastery over armament haki.
By coating his fists with Haki, Garp was able to demolish mountains with ease, which is a very impressive Haki feat. Even though he is past his prime, Garp has shown that he could injure Marco, and he was confident that he could have killed Akainu at Marineford.
Garp is easily the most powerful Vice Admiral in the Marines, and rivals or exceeds the power of the known Admirals and Fleet Admirals. After then-Admiral and future Fleet Admiral Sakazuki (also known as Akainu) killed Ace, Fleet Admiral Sengoku held Garp down to prevent him from killing Sakazuki in revenge.
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.
Garp would ultimately come out as the winner against Akainu. If the two of them fought at full power at the peak of their abilities, Garp's experience and his “D.” would ultimately help him overcome Akainu's Devil Fruit power, which is his strongest weapon. In all other aspects, Garp is the more powerful of the two.
Akainu's weaknesses are drowning and caught off guard by Seastone.
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. He's previously fought against the likes of Rocks, and even Gol D. Roger.
The answer is Shanks. In no way is a single admiral enough to take down a freakin yonko. An old, half dead whitebeard almost killed akainu and shanks is in his prime right now and it would be suicidal for akainu to go one on one with Shanks or any of the other yonko.
While in this form, Sengoku can unleash enormous shockwaves. Normally with such a powerful devil fruit, the user would best anyone in a fight, but Garp's insane haki makes up for his lack of devil fruit. The fight between the two legendary Marines would be close, but Garp would come out on top almost every time.
No he can't. Akainu is probably on another level of strength as Kaido or Big Mom. I know that a lot of people would disagree here, but everything redirects to that. From the world government side, Akainu will probably be Luffy's final villain.
he can beat those two yonko for sure. His power and haki and stamina are way too strong for these two yonkos.
Having been a Marine for most of his life, Garp has fought against and crushed countless pirates and even played a part in the defeat of the infamous Rocks. Garp was recognized by Roger to be powerful enough to fight him and it is quite evident that he is strong enough to fight any of the Yonko as well.
Issho possesses Haki, as do all admirals.
Jenny's sudden fame plays a big part in Garp's fear of violence. After all, there really are people out there who want Jenny dead. It's easy to forget that Garp grew up in relative isolation: It's just him and Jenny in the early days.
Garp is considered the strongest marine. Even in his old age, his power and strength remain intact. Garp's abilities could throw even the most powerful One Piece enemies on their backs. This grandfather and his monstrous punch will render any Devil Fruit users defenseless.
In his prime, Garp was also a worthy adversary to Gol D. Roger, the Pirate King, considered to be the only Marine who could match Roger's strength. Additionally, Garp's Haki is incredulous; he was able to crush mountains with his fists. He even managed to flatten the head of Don Chinjao with just one punch.
Sengoku adopted Donquixote Rocinante (Corazon) as his son after the death of his biological father by the hand of his brother.
Sengoku possesses a Mythical Zoan Devil Fruit that allows him to turn into a Buddha and attack using shockwaves. To date, he is the only Marine with the power to use all the three Haki types. Undoubtedly, with these abilities, he is capable of rivaling the Yonko.
Sengoku had somehow found out that Garp had adopted Portgas D. Ace, the son of Pirate King Gol D. Roger, which was meant to be kept secret from the World Government.
In One Piece 1055, Oda all but answered the question of who is stronger between the Yonko and the Admirals and the answer to the question is the former, and by quite a mile.
Devil Fruit users who can use Haki can resist the effects of Haki-imbued attacks, Akainu was able to take Haki-imbued attacks while maintaining his intangibility by extending his Haki through his body in order to nullify his opponents bypass.
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.
Garp. Receiving the title of 'Hero of the Marines' in the process, Garp gained immense fame by taking down Rocks. Just like Roger, it is likely that Garp became far stronger than he was during the God Valley incident, meaning he would've been at his prime in his later years.
He eventually joined the Marines in the hopes of becoming a strong soldier, and went to train under Garp and rise to the rank of Captain. He has a long way to go before he can fight against the likes of Luffy, Law, or any of the others. With his current progress, he will never surpass Garp.
He demonstrates proficient usage of Busoshoku Haki, being able to injure Luffy several times, bypassing his rubber body. He was also seen crushing Chinjao's uniquely shaped head with a Haki imbued punch 30 years ago, by hardening his fist and turning it obsidian black to increase the damage inflicted.