This page of the guide to Final Fantasy 7 Remake has a walkthrough for the Subterranean Menace side mission available in Chapter 14 - In Search of Hope.
The Subterranean Menace mission has to be completed in order to complete Secret Medicine. One thing you have to do is defeat the Type 0 Behemoth.
This giant monster is powerful, dangerous and fast. Replenish your health before starting the mission.
Remember to attack your opponent from the back or the sides, otherwise, he will attack you with his claws and hit the ground with all his strength to crush them with his horns. It is best to weaken the opponent with tranquil or ice spells. That way you can more easily knock him out, and at times like that, you can focus all your team's strength on inflicting massive injuries.