`Reverse` Dictonary.cs

var dic = new Dictionary<string, int>
      {
          {"Level_01", 1},
          {"Level_02", 2},
          {"Level_03", 3},
          {"Level_04", 3}
      }
  ;
  var x= dic.Select(kvp => new KeyValuePair<int, string>(kvp.Value, kvp.Key));
  var y = dic.ToLookup(pair => pair.Value, pair => pair.Key);

  new List<object>{x, y}.ForEach(result => result.Dump());