Actually, I already explained this back when Akame learned Haki.
But I found that some readers either skipped chapters or something and brought up this question again.
So I'll explain it in a separate chapter here.
In the official One Piece novel starring Ace, ONE PIECE Novel A, whose content was reviewed by Oda.
(It seems to have been adapted into a manga already, and there are even rumors online that it will be animated.)
In the latter half of the story, Ace, who had only recently set sail and formed the Spade Pirates, awakened Armament Haki on his own during a fight with a Vice Admiral named Draw.
"Why can you use Haki too?!"
"How the Demon should I know?!"
That was their brief exchange.
See?
Armament Haki did not actually require someone to teach you. In fact, most powerful fighters outside official organizations awakened it themselves while fighting people who could use Haki, provided they had the talent.
Both Akame and Hashirama Senju could already release their presence and sense the presence of others.
All that remained was for them to understand Haki through combat contact and become familiar with gathering it. They already had a greater advantage than most people on Sea Star, who initially knew nothing at all about Haki.
And as for talent, how could Akame and Hashirama Senju possibly lack it?
Also.
Ace's fight with Vice Admiral Draw did not even last an hour, while Hashirama Senju only learned Coating and Hardening here after fighting Aramaki for three days.
Was that fast?
I thought I had already been very conservative.
If I truly followed Oda's setting, Hashirama Senju should have directly learned Armament Haki Coating within half an hour of fighting Aramaki, then mastered Armament Haki Hardening within two hours at most.
That is