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This is IGN's page that details everything about Genshin Impact's weekly bosses. This includes the full guide for each boss fight present in Genshin Impact as well as the rewards that you might potentially get after defeating them.
Just like the name suggests, these bosses can only give you rewards once a week. However, they can be challenged/defeated countless times.
These bosses are harder than the others that's why if you're having a hard time clearing these bosses on your own, co-op might be a good idea.