One key element is Bill's unique skills. He's worked with dangerous magical creatures and has a lot of knowledge about the darker parts of the wizarding world. This knowledge is often used to keep Hermione out of harm's way. For example, if there are dark wizards using forbidden magic related to magical creatures, Bill can counter it.
The threat itself is a crucial element. It could be anything from dark wizards who want to use Hermione's intelligence for their own evil plans to ancient magical curses that target her. The nature of the threat determines how Bill protects her. If it's a curse, he might search for ancient counter - spells. If it's dark wizards, he'll use his combat and defensive magic skills. Also, the setting of the fanfiction plays a role. Whether it's set in Hogwarts, the Weasley's home, or some other wizarding location, it affects how Bill can protect Hermione and the challenges they face.