Buddha taught that everything is ephemeral and everything is grief. What a seeker should do in the above circumstances?
Buddha taught all worldly things are ephemeral but did not teach everything is "grief". "Grief" is an emotion created by the mind rather than a characteristic of things.
The seeker keeps reflecting & examining how all worldly things are ephemeral until the mind realises completely that ephemeral things cannot be any other way or 'suchness' ('tathātā').
This will result in the mind giving up desires & expectations towards ephemeral things.
This is described at the end of the Anatta-Lakkhana Sutta, as follows:
O monks, the well-instructed noble disciple, seeing thus, gets wearied of form, gets wearied of feeling, gets wearied of perception, gets wearied of mental formations, gets wearied of consciousness. Being wearied he becomes passion-free. In his freedom from passion, he is emancipated.