A Robust Fungal Nucleus can be used as a piece of Ascension Material for weapons in Genshin Impact. More specifically, the Nuclei play a role in ascending three different bows - the Hunter's Path, the End of the Line, and the King's Squire - to Ascension levels five and higher. As these Robust Nuclei are needed in high quantities to expedite the Ascension process, it's best for you to know where, and how, to efficiently farm them.
Like other Fungal Nuclei, the Robust Fungal Nucleus exclusively drops from defeating fungi-based monsters. As a four-star rarity item, the Robust Nucleus only drops from fungi of level 60 or higher. The following enemy types fit this criteria.
Besides their apparent fungal ties, what all of these enemies have in common is that they all share residence in the Underground Mines of the Chasm. These mushrooms all tend to congregate in three particular locations within the Chasm - the easternmost portion of the Underground Waterway closest to the main section of the mines, the southern alcove of the Stony Halls, and along the length of the Glowing Narrows.
Related: Genshin Impact: Hunter's Path Bow - How to get it, stats, and ascension materials
Getting an item as rare as the Robust Fungal Nucleus to drop as often as its high-tier Ascensions require can be a daunting and grindy task by itself. Fortunately, Alchemy can be performed on the Fungal Nucleus drops of lower-tier fungi monsters to boost the item up to its next tier. To this end, you can create a single Robust Nucleus using three Dormant Fungal Nuclei and 125 Mora each.