I have my Guardian's cure at the first repeat slot in it's skill list (the one-shot slots are empty). To activate your Guardian's Cure, you need to have you HP below 20% of its maximum, but you also have to trigger your Guardian to attack, by either attacking an enemy yourself without killing it, or right-clicking an enemy. By triggering your Guardian to attack, only then will it use Cure.
I bought a MyShop Rune that triggers my Guardian to attack when I am being attacked myself. That has been working really well for me, auto-curing me when I am damaged and my remaining HP is below 20%.
tldr: Trigger your guardian to attack
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