EnemyController.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class EnemyController : MonoBehaviour {
	//structures
	enum Direction {
		UP, LEFT, RIGHT, DOWN,
	};

	enum Behaviour { //NOTE: These need better names than just "first" and "second"
		FIRST, //move up/down at targetX
		SECOND, //move towards and stay at target Y
	};

	//members
	public Vector2 moveSpeed = new Vector2(200, 200); //NOTE: doesn't HAVE to be a vector
	public Vector2 target = new Vector2(3, 3); //NOTE: doesn't HAVE to be a vector
	public Direction direction = Direction.LEFT;
	public Behaviour behaviour = Behaviour.FIRST;

	//behaviour parameters
	public bool moveUp = true;

	//components
	Rigidbody2D rb;

	void Awake() {
		rb = GetComponent<Rigidbody2D>();
	}

	void FixedUpdate() {
		HandleMovement();
	}

	void OnBecameInvisible() {
		//cleanup
		Destroy(gameObject);
	}

	//movement control
	void HandleMovement() {
		switch(behaviour) {
			case Behaviour.FIRST:
				HandleFirstBehaviour();
				break;

			case Behaviour.SECOND:
				HandleSecondBehaviour();
				break;
		}
	}

	void HandleFirstBehaviour() {
		//if passed target.x, move up/down
		Vector2 velocity = new Vector2(-moveSpeed.x, 0);

		if (rb.position.x < target.x) {
			velocity.y = moveSpeed.y * (moveUp ? 1 : -1);
		}

		rb.velocity = velocity;
	}

	void HandleSecondBehaviour() {
		//if not at target.y, move towards target.y
		Vector2 velocity = new Vector2(-moveSpeed.x, 0);

		if (rb.position.y < target.y && moveUp) {
			velocity.y = moveSpeed.y;
		}

		if (rb.position.y > target.y && !moveUp) {
			velocity.y = -moveSpeed.y;
		}

		rb.velocity = velocity;
	}

	//NOTE: could add more functions down here for different directions
}