Who might be responsible for Katniss getting shot in the fanfiction?

It could be a Sith or a Sith - inspired enemy. They are often the ones causing trouble for the Jedi, and shooting Ahsoka would be a way to strike at the Jedi Order.

Maybe it's a human who has somehow discovered the world of demigods and is afraid of their existence. They shoot Annabeth out of fear and misunderstanding. Another option is that it's a rogue demigod who has gone bad and is working against their own kind. This character could have a personal grudge against Annabeth specifically, perhaps because of her intelligence and leadership skills which they envy.

It could be a rival agent from another agency. They might see Annie as a threat and want to take her out of the picture.

It could be a random enemy, like a rogue monster that manages to catch her off - guard.

It could be an enemy that manages to sneak onto the Argo 2. Maybe a monster that they thought they had defeated before but actually just bided its time and then attacked Percy when he was least expecting it.

It could be a mischievous Pokemon. Maybe a Ditto that has gone a bit wild and accidentally turned into a Pokeball and then somehow 'captured' Ash when he touched it.

It could also be an ancient prophecy that is misinterpreted. The Oracle might give a prophecy that the gods misunderstand as Percy being a threat. So, in an effort to prevent the prophecy from coming true in what they think is the wrong way, they disown him.

There's a possibility that it's an old enemy who has somehow regained power or found a new way to attack. For example, Frieza's family or the Red Ribbon Army if they had some kind of resurgence. They might target Vegeta and Bulma out of revenge or to gain access to Capsule Corp's technology which Bulma controls.

A new and powerful rogue ninja could be responsible. This ninja might have a vendetta against Konoha and see Naruto as a symbol of the village, so they target him and use their strong and devious techniques to injure him badly.

Another possibility is a jealous student at Hogwarts. There are always students who might be envious of Hermione's academic achievements. They could set up a prank or a trap that goes wrong and ends up hurting her. For example, a student who is tired of Hermione always getting the top grades in Potions might tamper with her cauldron during a class experiment, resulting in an explosion that hurts her.