Sundered

As it says, kill every miniboss.

Each area has 3 of them, beating all 3 will give you an Elder Shard (1 fragment per)

The ones in the Cathedral can be annoying to find (as they're mostly off the main screen), but the room connections on the map can help guide you.

I advise you use the map and just explore all (or at least most of) the rooms in each area.