The sword could attract dark forces. Since it's a powerful weapon, it might act like a magnet for dark wizards or evil creatures. Harry then has to constantly defend himself and the sword from these threats, which can be really difficult especially if he's not fully trained in sword combat.
Well, it could be a magical sword like the Sword of Gryffindor. In many fanfics, authors might choose this iconic sword as it has a strong connection to Harry through Gryffindor. It's a powerful weapon against dark magic and has a lot of history within the Harry Potter universe.
In the fanfic, the main challenges for Harry could be related to the limited time he has. He has his own studies and tasks to attend to, like preparing for exams or dealing with the mysteries of the Chamber of Secrets. Yet, he still has to find time to protect Colin. Also, the magical environment itself can pose challenges. There could be spells that are difficult to counteract while protecting Colin, especially if they are new or dark magic.
One challenge could be controlling the intensity. Lightning is extremely powerful, and if he doesn't have proper control, he might end up causing more harm than good. Another might be the unpredictability of the element itself. Lightning doesn't always behave as expected in nature, and it could be even more difficult to manage magically. Also, he may face opposition from those who fear his power or think he's becoming too powerful.
One challenge could be controlling the assassin's violent tendencies. Since the person is being groomed to be an assassin, it's likely they have a natural inclination towards violence, and Harry has to find a way to temper that while also teaching self - defense.
One challenge could be dealing with the traditionalist teachers and pure - blood families. They might resist any changes Harry tries to make as they are used to the old ways. For example, if Harry wants to introduce muggle - born friendly policies, some of the old - school pure - bloods might oppose it.
One challenge could be controlling the intensity of the ice. If he makes it too cold or creates too much ice, it might harm his friends or the surrounding environment. For example, if he's in a room and loses control, he could freeze everything and everyone in it.
One challenge could be the basilisk's own will. Even if Harry has the ability to control it, the basilisk is a powerful and ancient creature with its own instincts. It might resist Harry's control from time to time, especially when it comes to situations that go against its natural behavior, like being made to not attack innocent students.
One challenge could be controlling the intensity. Lightning is a very powerful force, and it might be difficult for Harry to rein it in. He could accidentally cause too much destruction when he just wants to create a small spark.
One challenge could be the unfamiliarity with the Fairy Tail world. Different rules of magic and different types of magical creatures might be new to Harry. Another is that the Fairy Tail characters themselves might be very powerful and have their own unique abilities that Harry has to figure out how to counter.
One challenge could be resistance from the existing authority. There might be some teachers or administrators who don't want to give up their power easily. They may see Harry as too young or inexperienced.