Code

// GETS THE BOUNDA WHERE MONSTER CAN MOVE
Vector3 ScreenSpace = new Vector3(0.95f,0.1f,1.0f);
Vector3 ConvertedVector3 = Camera.main.ViewportToWorldPoint(ScreenSpace);

// GET X AXIS MIDLE POINT
float MidScreen = ConvertedVector3.x /2;

// GET CURRENT POSITION OF THE MONSTER
Vector3 CurrentPositionVector = transform.position;
Vector3 CurrentConvertedPosition = Camera.main.ViewportToWorldPoint(CurrentPosition);

// POSITION OF THE MONSTER IN SCREEN SPACE
float CurrentPosition = CurrentConvertedPosition.x;

// MOVE THE MONSTER
if(CurrentPosition < MidScreen) {
	// MOVE LEFT
} else {
	// MOVE RIGHT
}