“Tobi, since you like them so much why don't you train them?”

“Oi old man, are you serious? I get to beat these brats up similarly to how you did with me? LETS GO, sorry kids nothing against you guys or anything, but since i can't be the old man guess ill have to deal with his grandkids instead.”

Tobi runs straight towards both boys whilst unsheathing his sword, he does a vertical slash towards the older one who hardly blocks it with his metal pipe, but is pushed back a few meters

“Gomu Gomu no Pistol.” A hand stretched out but missed due to Tobi slightly shifting his head due to his observational haki, the young man then grabs the stretched out hand and puts all of his weight into one foot as he pulls on the hand sending Luffy flying towards Ace and Tobi. As the flying rubber man gets nearer to the duo. A metallic black coating appears on Tobi’s fist and he sends it down straight onto Luffy’s head.

After knocking Luffy to the ground, Tobi spins around on the foot planted in the ground before sending a kick towards the shocked Ace. “Damn old man why does it feel so satisfying giving out the love punches? Is that why you always do it to me?”

“Argh.. how can you hurt me? I'm supposed to be a rubber man” The little kid asked whilst rubbing his head. “Kid, just remember this, just because you’ve eaten a devil fruit doesn't make you invincible, out on the grand line the amount of insane and crazy things are unbelievable that you being a rubber man wouldn't even bother many.”

Ace got up and ran towards the back of Tobi and started swinging the pipe without any rhythm and is just swinging the pipe randomly. Tobo just slowly walks back and does light swings deflecting the pipe and dispersing most of the power behind each swing 

Whilst Ace and Tobi were fighting Luffy sat up and tried to sneak up behind Tobi just to get another punch on the top of the head sending his face first into the ground. Tobi looked towards Ace and gave a smirk before sprinting towards Ace after the pipe got deflected again.

You could see the fear in his eyes as he got closer as if he could see death standing behind Tobi, Ace lost all his strength in his legs and flopped to the ground. Luckily for Ace. Tobi's fist went straight over Ace's head. Luffy managed to get back up again and grab Ace from afar and barely managed to get him away. Just before a fist smashes straight into the ground sending a spider web crack in a diameter of 10 metres. Causing both Ace and Luffy’s backs to be drenched in cold sweat.

"Damn Garp have you even been training them or are you doing your horrible way of training, like you did to me?" Tobi complains about Garp's horrible way of training. "Why are you complaining runt you got strong with my training right?"

"Only because I was training myself everyday on my own. If I just followed what you did, then I would have had a good battle instinct but that's about it. What's the point of good battle instincts if your attacks don't have enough power behind them."

“Oi old man tell Sengoku I'm taking a year's vacation, I'm gonna spend a year training these two brats so they don't get themselves killed so easily when they enter the grand line.” Tobi stated to Garp. “you know he's not going to be happy about that right?”

“Eh if i've learnt anything about you and Sengoku is that you dont give a fuck and he just relents since he cant do anything about it, plus he knows its better for you to be away from the marines some times.” Tobi says which earns him a smack to the head, before Garp starts walking back to the ship.

“Luffy get here now, your devil fruit is strong and has a lot of potential but the way you fight gives you a lot of openings, come use that stretchy ability on me again.” 

“Gomu Gomu no Pistol” the fist shoots straight towards Tobi who slightly dodges the attack and then gives a light punch on Luffy’s arm, which stops all of his momentum giving Tobi the advantage. “See Luffy you sent 1 punch towards me and i've already stopped your attack and gave myself an advantage”

“How do you always manage to dodge even when you're not looking at me?”

“Oh that, Ace come over here as well, this will be useful for you as well.”

Once both kids are sat in front of him he begins speaking. “In this world there are 3 types of Haki, 2 of which every living being can use; these two are observational haki and armament haki. Observational haki is like a sixth sense in a sense it makes a domain around the wielder which allows them to perceive things that you most may not be able to, depending on their proficiency the larger the range you can perceive things at. At the utmost limit some people are able to see the future for a few seconds which makes fighting them even more difficult.

On the other hand, armament haki is like a portable piece of armor that can help you defend against attacks as well as make your attacks more powerful, it can also allow you to coat your weapons but that's a lot more difficult. Before I continue I've got a question for the 2 of you. Do you know the difference between the 3 types of devil fruits?”

“Nope” both boys shake their heads.

“In the world the 3 types of devil fruits are zoan, which allows the user to transform into an animal and take on different types of traits, an example of this is the  neku neku no mi which allows the user to transform into a type of cat, and gains the agility of a cat. Next is the paramecia type which has the widest range, this type of devil fruit can affect nearly anything, such as the body like Luffy’s gomu gomu no mi which has transformed him into a rubber man, paramecia types can also affect other things such as gravity. Lasty is the most troublesome in the first half of the grandline, logia type devil fruits which can affect the users body turning them into the material/element of their fruit, an example of this is the mera mera no mi which transforms the user into literal fire. But keep in mind that there could always be exceptions regarding devil fruits.”

“Okay but why are logia users the most troublesome in the first half of the grand line?” Ace asks.

“That is because in the first half of the grandline most people don't know about armament haki, aside from allowing the user to have a stronger defense as well as attack power, armament haki allows the user to hurt devil fruit users, an example of this is me being able to hurt luffy despite him being a rubber man.

Finally the last type of haki is called conquerors haki, the biggest difference between the 3 is that only 1 in 100000 are able to use it, another name for the conqueror haki is the will of the kings, it is the manifestation of the users will which allows people to affect others souls, which can knock people out and for the weaker people they could out right die just by the will of someone,