Luffy uses Gear 5 in a significant battle towards the later part of the manga. But the exact chapter or arc can vary depending on how you're reading it.
Percy often uses Observation Haki to anticipate the attacks of his enemies. He can sense their movements and intentions, which gives him an edge in combat, especially against fast - moving or sneaky foes like some of the monsters in the Percy Jackson world.
Naruto's use of haki would be a game - changer for his combat skills. Haki has different forms. For example, the Observation Haki could allow Naruto to see the future movements of his opponents to some extent. This would combine really well with his already amazing combat skills like his Rasengan and Shadow Clone jutsu. He could use his clones more effectively to surround and attack his enemies while using haki to predict their counter - attacks.
One possible storyline could be Naruto discovers he has Conqueror's Haki during a battle with a powerful enemy. This new power gives him an edge and he uses it to inspire his allies and intimidate his foes. For example, when facing the Akatsuki, his Haki could make some of the weaker members hesitate in their attacks.
It could be that his training in magic somehow awakens his haki. As he delves deeper into advanced magic spells and combat training at Hogwarts, his body and mind start to develop this new ability. For example, during a particularly intense dueling session, he feels a strange energy coursing through him which he later realizes is haki. His professors might initially be baffled by this new power, but as he learns to control it, it becomes an asset in his battles against the forces of evil.
In this kind of fanfiction, usually the plot might involve Luffy starting to discover his latent conqueror's haki power. Maybe he first experiences some intense battles where he unknowingly shows signs of it. Then, he might meet an old and wise character who recognizes this power in him and starts to guide him on how to control and use it effectively.
Aside from Luffy, characters like Ace might show up in this type of fanfiction. Ace, being Luffy's brother, could have some connection to Luffy's discovery of conquerer's haki, perhaps through memories or some form of spiritual guidance. Additionally, some powerful enemies might be present. These enemies would serve as the motivation for Luffy to quickly master the conqueror's haki in order to defeat them and protect his crew. There may also be some minor characters from the One Piece world who assist Luffy in his learning process, like the inhabitants of a special village that has knowledge about haki.
Naruto would become more confident. With Conqueror's Haki, he would know he has an extremely powerful ability. This would boost his self - assurance in battles and in his leadership role. For instance, when leading his team, he would be more assertive.
Well, there are 'luffy haki fanfic' stories that focus on the development of his Observation Haki. In some of them, Luffy is able to predict the moves of his opponents far in advance, like being able to dodge attacks that are seemingly impossible to avoid. These fanfics often add new elements to the use of his Haki, like being able to sense the emotions behind an opponent's attack as well.