You could make use of a game controller to count the number of enemies in your scene by having them register with the controller when they enable and unregister when they disable. When you reach a count of 0, you can load a new level.

To do this, find or create an empty game object in your scene tagged GameController (by default, there is none).

On this game object with the tag GameController, add the following ObjectivesManager.

ObjectivesManager.js

private var objectives : int;

function OnEnableObjective() {
    objectives++;
}

function OnDisableObjective() {
    objectives--;
    if (objectives == 0)
        SendMessage("OnObjectives");
}

function OnObjectives() {
    Application.LoadLevel(Application.loadedLevel + 1);
}

Then, on every enemy you should have code like this. You want to let the controller know that an objective was enabled and disabled at key moments.

Objective.js

function OnEnable() {
    var gameController = GameObject.FindGameObjectWithTag("GameController");
    gameController.SendMessage("OnEnableObjective");
}

function OnDisable() {
    var gameController = GameObject.FindGameObjectWithTag("GameController");
    gameController.SendMessage("OnDisableObjective");
}

function Update() {
    var enemies = GameObject.FindWithTag("enemy") //find gameobjects with the tag "enemy" (you must therefore tag the enemies with "enemy"
    if(enemies == null){ //if no gameobjects with the tag "enemy" are found

    //change scene

    }
}

not certain this will work as I'm away from unity at the moment but basically it finds all the gameobjects with the tag "enemy", if it doesn't find any then you can put in a script to change the scene

hopefully its useful

Scribe