How to defeat the Guardian Golem in Elden Ring Nightreign

If you're enjoying the tanky birdman, Guardian, in Elden Ring Nightreign, then you'll soon start building out his journal entries. In chapter four, you'll unlock his first remembrance mission where the Roundtable Hold's caretaker sends you searching for books about cursed weapons, requiring you to defeat the Guardian Golem in Limveld.

I'd expect no less from FromSoftware, but this personal objective is missing something: directions. It's not immediately clear what the Guardian Golem even is, let alone where to find it.