Q: Why is Natsu able to kill so many dragons - but then gets wrecked by Gildarts?

A: Natsu uses Dragonslayer Magic, a magic that is utilized specifically to deal with dragons. Thus, unlike other offensive spells, his damage output is amplified and actually deals damage to dragons. Dragons feel over 5 x of what other creatures, such as wizards would feel from his attacks. This is why some of the spells he used to defeat other dragons may not have an effect that is as large on other mages.

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Q: Why is Natsu so afraid of Erza and Mira? He's clearly more powerful than them.

A: When he was younger, Natsu was brought up by dragons. His only social interaction was with Igneel and Acnologia. He's learned to not fear dragons because he's grown up amongst a few. He's had little interaction with humans, aside from those in his village life. Erza is a very scary woman. His initial fear comes from her displayed aggressiveness and established dominance over other children and himself. We do know that later on he feels less scared of her, as of Ch 10

As for Mira, he's not really afraid. Their relationship is more of a "teasing" one. Think of it as Natsu and Lucy from canon, but he's more emotionally dependent, and Mira is a bit more of her sadistic old self.