Trying to create gameobjects at intervals [C#]

Unity Engine
1

Hey guys,
my current challenge is trying to instantiate “enemy” gameobjects at 2 second intervals. However, when I try to use the “waitForSecond” type function, they are simply instantiated MANY TIMES and nearly crashes unity. Ideally, my (pseudo)code would look like this:

void Update()
{
waitForSeconds(2);
gameObject.instantiate(“Enemy”);
}

2

that is what i use for the moment

using UnityEngine;
using System.Collections;

public class WaveSpawningScript : MonoBehaviour {

    public  GameObject     spawn_point;
    public  int         wave_size     = 5;
    public     int            mul = 1;
    private float         spawn_rate     = 2.5f;
    private float         next_spawn     = 0.0f;

    public void Update_()
    {
        if(Time.time >= next_spawn && wave_size != 0)
        {
            SpawnMonster();
        }
    }

    private void SpawnMonster()
    {

            next_spawn =  Time.time + spawn_rate;
            wave_size --;
            GameObject monster = Instantiate(Resources.Load("Monster/Slime/SlimeMonster"),spawn_point.transform.position,Quaternion.AngleAxis(180,new Vector3(0,1,0))) as GameObject;
            monster.GetComponent<Monster>().life = 20 * mul;
    }
}

Update_ is called from the main loop when needed

3 
InvokeRepeating
4

Thanks for the help, guys!