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 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.
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.
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.
Gildarts would probably win. He's one of the strongest mages in Fairy Tail. His magic is very versatile and his strength is off the charts. Natsu is strong, no doubt, but he's still young and has a lot to learn. In fanfiction, the writers might be tempted to make it a close fight, but realistically, Gildarts' skills and power level are much higher. He's faced much tougher opponents and come out on top.
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.
Lucy might also be an ally. She has her celestial spirits that could be a great help. For example, some of the spirits like Leo could be very powerful in combat against the dragons. Also, Erza would be a great addition. With her various armors and combat skills, she could hold off a large number of dragons while Natsu focuses on the more powerful ones among the 10000.
Well, perhaps Natsu uses his powerful fire magic to take on the dragons one by one. He might start by finding a strategic location to face them, like on top of a large mountain where he has a better view. Then, he'd use his signature moves like Fire Dragon's Roar to blast groups of dragons at a time.