Harry could use his intelligence and the skills he's learned from his friends. He might outwit the bullies by setting up traps or cleverly avoiding them. For instance, he could use the Marauder's Map to know their whereabouts and stay clear.
He might try to reason with the Gryffindor bullies at first. Since Gryffindors are supposed to value courage and justice, he could appeal to their better natures. He could show them how their actions are wrong and against the Gryffindor spirit. If that doesn't work, he may seek help from his closest friends like Hermione and Ron to stand up to them together.
Harry could also focus on his own growth and strength. He could practice more advanced magic in secret, so that when the bullying gets too much, he can defend himself. He might also find allies outside of Gryffindor, like some Hufflepuffs or Ravenclaws who see the injustice and are willing to support him.
Another way Harry could deal with it is by getting the support of the Hogwarts faculty. He could gather evidence of the bullying, like spells cast at him without provocation or mean - spirited remarks. Then, he could present this evidence to a trusted teacher, such as Professor McGonagall. She would likely take action against the bullies, perhaps giving them detention or restricting their privileges. This would not only stop the bullying but also show that there are consequences for such actions at Hogwarts.
He might face a lot of scrutiny from the wizarding world. Some might see him as a hero, but others could be afraid of his new - found power. So, he would have to deal with political issues within the Ministry of Magic.
Harry would probably be deeply hurt. He might pour himself into his studies or his work in the wizarding world. He could also try to distract himself by taking on more dangerous tasks or missions. But ultimately, he'd have to face his feelings towards Ginny and decide whether to forgive her or move on completely.
He could use spells to restrain it. For example, he might put a binding spell on the Basilisk so it can't move freely unless he allows it. This way, he can control the danger it poses.
He might go to Madam Pomfrey in the hospital wing. She has all kinds of potions and remedies to help with ailments, so she could easily give him something to soothe his stomach.
He might focus on his studies. In the wizarding world, there are so many interesting magical subjects like Potions, Transfiguration. By burying himself in books and learning new spells, he can at least distract himself from the lack of friends.
He could also react with confusion. Natsu is a bit of a simple - minded character in some ways, and he might not understand why someone would want to bully him. This confusion could lead to him trying to figure out the bully's motives.
Later, he could become depressed. Harry has always valued his friends highly, so losing them would be a huge blow. He might isolate himself in his dormitory, not participating in school activities as much. He would probably be constantly thinking about what went wrong and how to fix it.
He might first try to calm the owl down. He could use a soft voice and gentle magic, like a simple soothing spell. Then he'd carefully examine the injury. If it was a broken wing, he'd try to splint it with a small stick and some cloth.
Well, in some fanfictions, he might be healed by a powerful magical herb that is rarely found. Maybe a character like Hermione goes on a dangerous quest to find it. She uses her extensive knowledge of magic and ancient texts to track it down, and once she gets it, the herb has the power to mend Harry's severe injuries.