So Gu Shanhai could only honestly choose a small elemental core; he didn't want to take risks.
The quality of this elemental core is merely that of a sacrificial offering. Getting it is enough; there's no real need to get one from an Element Elder. A small one can already reach the limit of an elemental blessing.
It is just a test for an apprentice, after all. Expecting an apprentice to kill an Element Elder? That's absurd.
The main point is to test the apprentice's survival capability as well as their bravery, and killing a small elemental creature is already sufficient to represent that.
If it wasn't for the major changes in the world, entering the Elemental Realm to survive on one's own and then returning wouldn't be a big problem at all. In their heyday, the Marked Druids were not something the Elemental Realm could provoke. A Legendary Druid alone could cause the entire Elemental Realm to collapse.