Master Trainers can be received as a reward from Legends of Kalevala Seasonal event.
There are reports that they are receivable from Hero Academy. level 7.
Staff confirmed that receiving them from HA 7 should not be possible.
They do no have a specific element, and they can not be used in battle so they can be used for feeding other heroes only.
They give the same XP during leveling regardless of the element.
Regardless they have a Special skill called “Traner Strike”, which is the follwoing:
As they aren’t meant to be out till v49 and was no mention of them going into HA I am expecting this to be confirmed as a bug and them removed from HA in same way when ninja troops first came out there were a few cases of them being received from HA due to an error