Another solution that you can use is log in a second offline gamertag as player 1 (we'll call him player 1) and then log the gamertag that you want the achievement to go to in as a second controller. Begin the game regularly starting with only player 1 playing use him to get as far as possible. You should be able to use him to get to at minimum the start to middle of the last level. As soon as you are down to your last life and are near death press start on your second controller and finish the game with the gamertag that you wanted the achievement to show up on. So in essence doing the game this way you basically start with six lives. This method works I have seen it done but I just can't quite get far enough into the last level and I am always just 1 life short of killing the final boss. Given another week of practice this achievement will be mine.