public class Game1 : Game
{
  [...]
  private void CheckAndRemoveEnnemy()
  {
      for (int i = 0; i < _ennemyShips.Count(); i++)
      {
          if (_ennemyShips.ElementAt(i).Rectangle.Y > graphics.PreferredBackBufferHeight || _ennemyShips.ElementAt(i).Life <= 0)
          {
              if (_ennemyShips.ElementAt(i).Life <= 0)
                  _score++;

              _ennemyShips.Remove(_ennemyShips.ElementAt(i));
              continue;
          }
      }
  }
  [...]
}