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.
One interesting plot in Naruto Armament Haki fanfiction is that the Akatsuki organization somehow gets hold of the knowledge of Armament Haki and starts to train their members in it. This completely changes their power levels and their goals. For example, Pain could use Armament Haki to make his attacks even more devastating. Meanwhile, the Leaf Village has to scramble to understand this new ability and find a way to defend against it. There could also be a plot where a young, unknown ninja has a natural affinity for Armament Haki and is hunted by various factions who want to use his power for their own ends.
In some fanfictions, Naruto might discover haki during a particularly intense battle. Maybe when facing an enemy with an unusual power that his regular jutsu can't handle, he unknowingly taps into his latent haki ability.
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.
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 way to incorporate Armament Haki into Naruto fanfiction could be through a crossover concept. Maybe a powerful jutsu creates a rift that brings in elements from the One Piece world where Haki exists. Naruto characters could then start to develop this ability. For example, Naruto, with his strong willpower, might be able to master Armament Haki quickly. It could add a new layer of combat abilities to the existing ninja skills in the Naruto story.
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.
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.