After dislodging the giant skeleton’s eye with a precision arrow shot, they swap the wooden spear they have equipped for a stronger metal sword in order to destroy the eye and kill the Stalnox.

Source: https://gamerant.com/zelda-breath-of-the-wild-player-stalnox-fight-instant-death/

Some of the Depths Stalnox will use Gloom-based attacks, so it's important to prepare with some Sundelion and Dark Clump food beforehand to keep Link's Hearts up.

Source: https://gamerant.com/zelda-tears-of-the-kingdom-all-hinox-stalnox-locations-hyrule-depths-totk/