Best Bud With Benefits

Stonehead is located at the gate of the ember ward, and is the objective of this achievement. Beside him is a Chewed Light Shard that starts the quest Snacks for Stonehead. To make the quest object interactable and get close enough to pick it up you must pick up the Hollow Rock behind him, just to the left of the gate on the inside of the Ember Ward.

Snacks for Stonehead requires you to gather some various materials across the Ember Ward-
Gooey Bug Bites from the nearby Ravenous Swarms,
Ash Crisps from Ash Crawlers,
Mirror Candy from mobs at the Shrouded Asylum
and 'A Light Snack', which you can get by picking up a Remnant of Light.
To complete the quest run up to Stonehead without the Hollow Rock disguise and allow him to pick you up, at which point you can talk to him to turn it in.

This achievement is awarded on completion of the quest and your character gains the buff Stonehead's Best Bud Benefits, which allows free passage past Stonehead.

For those with the TomTom addon:
/way 38 64 Hollow Rock (West)
/way 40.2, 65.6 Hollow Rock (East)
/way 39 65 Chewed Light Shard
/way 37 65 etc Ravenous Swarms - Gooey Bug Bites
/way 32 60 Remnant of Light
/way 31 61 Ash Crawlers - Ash Crisps
/way 26 52 Shrouded Asylum - Mirror Candy

BTW, you don't throw the Hollow Rock at Stonehead. Just keep it on as a disguise while youi talk to the light shard