Countdown increasing when you kill an enemy

If you are adjusting the amount of time the system will 'wait' for, It may be tricky to use yield WaitForSeconds. Instead it might be better to use an if statement looking at Time.realtimeSinceStartup and have a variable to hold the current time allowance and the start time. For example something like:

 var TimeToGo = 60;
 function Start(){
 var StartTime = Time.realtimeSinceStartup;
 While((StartTime + TimeToGo) > Time.realtimeSinceStartup)
 {      }
 Application.LoadLevel("Inicio");
 }

Then you put some code into your enemies that whenever they are killed they add 10 to 'TimeToGo'. This way, the system is always checking against the updated version of the time remaining.

I haven't tested this code so it may have problems, but i hope it gives you an idea of how to solve your issue.

This is my first answer: What you should probably do is using C# and this command:

    public class SecondsToSurvive : MonoBehaviour {
             public float Countdown;
             public int TimePerKill;
             public bool Kill;
             public bool Lose;
             void Start {
          Lose = false;
         //This bool must be true when the countdown reach 0.
               Countdown = 60;
             //Seconds
              Kill = false;
             //A boolean variable, we'll use it later. This boolean must be true
          // when you get a kill
              TimePerKill = 10;
             //Seconds added per kill
             }
           void Update () {
              Countdown -= 1 * Time.deltaTime;
 //Countdown will be decreased 1 time per every second.
              if(Countdown <= 0) {
           Lose = true;
 //If you have 0 or less seconds, you lose
         }
         if(Kill == true) {
           Countdown += TimePerKill;
 //10 seconds added when you got a kill.
            Kill = false;
 //Then, no more seconds are added.
         }
             }