It depends on how the author of the fanfiction has developed the plot and the characters. They could have given Natsu some special power - up or a unique situation that makes him stronger than Gildarts. For example, the author might have introduced a new form for Natsu that gives him an edge in combat, or perhaps they've weakened Gildarts in some way for the sake of the story. However, in the original series, Gildarts is extremely powerful, so this is a significant departure in the fanfiction.
In some fanfictions, Acnologia is made out to be Natsu's father. But this is not part of the original canon of the Fairy Tail series. Fanfiction writers often like to create such unexpected family relationships to add new twists to the story.
Well, one possibility is that Gildarts could share some of his powerful magic techniques with Natsu. Since Gildarts is a very experienced wizard, he might see the potential in Natsu and offer some tips on how to better control his fire magic, which is similar to Igneel's. Maybe he could also tell Natsu some stories about his own adventures that would inspire Natsu to become even stronger.
In many Fairy Tail fanfictions, the battle between Natsu and Acnologia is often depicted as extremely epic. Natsu's fiery determination and his unique dragon - slayer magic make for an interesting contrast against Acnologia's overwhelming power. Some fanfictions might have Natsu discover new forms or powers in order to stand a chance against the fearsome Acnologia.
A common theme in such fanfictions is Natsu's unwavering courage when facing Acnologia. He might not be as strong as Acnologia at first, but his will to protect his friends and the Fairy Tail guild drives him forward. His dragon - slayer magic could potentially have some hidden aspects that are only revealed during this encounter.
One possible plot could be that female Acnologia, being a powerful and feared being, initially sees Natsu as an enemy due to his association with the dragon slayers. But as they cross paths more often, she starts to notice his determination and the kindness in him. Eventually, they team up against a common greater evil, and during their adventures, a romantic bond slowly forms between them.
In the fanfiction, Gildarts could meet Natsu and Igneel during a time of crisis. Maybe their town is under attack. Gildarts would immediately jump into action, and Natsu would follow suit, with Igneel guiding Natsu from the side. Gildarts might be impressed by the way Natsu and Igneel work together, as if they share a single mind. They could communicate without words, and Gildarts would try to learn from their unique bond. After the battle, Gildarts would offer to train Natsu further, seeing his great potential enhanced by his connection with Igneel.
Natsu could try his all - out offensive with his most powerful fire attacks. He might start with his Fire Dragon's Roar and then quickly follow up with a combination of punches and kicks enhanced with his fire magic. Another strategy could be to use his environment. If they're in a place with flammable materials, he can set them on fire to create distractions or additional sources of power for his attacks.
One common theme is the exploration of Natsu's new - found heritage. Another is the power struggle within Natsu as he grapples with having the blood of a powerful and feared dragon like Acnologia. Also, there might be themes of redemption for Acnologia through his relationship with Natsu.