His friends are the most likely ones. They know his habits and routines well enough to pull off some great pranks. For example, if Leo is always punctual, they could set his alarm clock wrong to make him late.
Well, in this fanfiction, Leo could be pranked by having his clothes swapped while he's in the shower. So he comes out thinking he's going to put on his normal outfit but instead gets something completely outlandish. Another possibility is that his friends set up a fake lottery win and then tell him it was all a joke.
A common prank is changing the labels on her tea jars. So when she goes to make her usual tea, she ends up with a completely different flavor. It's a simple yet effective prank that can lead to some really funny reactions from Toriel.
In certain 'dumbledore gets pranked fanfiction', a popular prank is to make Dumbledore's office furniture float and move around chaotically. Imagine him trying to sit at his desk and the chair keeps scooting away or the desk starts doing a little dance. Also, there could be a prank where his glasses show false images, like seeing a dragon in the hallway when there's actually just a cat.
Mario and Luigi are the most likely. They are always looking for ways to outwit Bowser, and pranking him would be a fun way to get back at him for all his evil deeds.
Fred and George Weasley are very likely candidates. They are known for their love of pranks and their mischievous nature. They might see pranking Dumbledore as the ultimate challenge.
One interesting aspect in such fanfiction could be when someone pranks Toriel by making her think that her favorite recipe has been completely changed. They might switch out ingredients when she's not looking. Toriel then tries to cook with the new 'ingredients' and gets really confused, creating a humorous situation.
He might also find some allies. Leo can look for friends or even adults who can support him. If he has a good friend, they can walk together and the bully might be less likely to target him. And if he tells an adult like a teacher or a parent, they can take steps to stop the bullying.
Raph usually gets really angry. He blames himself for not being there to protect Leo or he gets mad at the enemy who hurt Leo. He'll be the first to want revenge and will be very protective of Leo while he's injured.