This achievement is very glitchy much like the 'Mission Accomplished' achievement.
If the "target weight" system isn't working at all, this achievement is impossible to obtain...
1. THE BUG)
For the first 4,8 or 12 week Routine, you are most likely to play it without being asked at the very beginning of it for a "Target Weight". Also you won't be able to list in any additional Calories burnt outside of the game... After every weigh in you almost score no points and show no signs of climbing to the top of the board against your AI rivals, thus either getting voted off the course or not coming 1st place in the final week... The reason being is because the game thinks you are losing nothing, due to the fact that you have a N/A goal, and the exercise you do is not enough and must be 20x more a day (which is something a doctor would advise you not to do for your own health).
2. THE SOLUTION)
From my experience, after I did a 4 week routine I decided to do a 8 week one which the game had on the list. The first question it had asked me was "What is your Target Weight Goal?". The bug seemed to clear up after I had done 1 Routine. : /
3. THE GUIDE)
When you are asked about your target weight, make it how ever high or low you wish (I went for something whack like 21.9kg).
With the "target weight" system working you should also be able to edit any additional calories burnt outside of the game, which is crucial to win this achievement seeing as it expects too much...
When you begin your first 20 minute workout (which the game forces you to do), you can just walk away and do something else until it's done. When the time is up and you have to edit your settings for the 8 week course (I'm not sure if the other courses work but you can always experiment), set it to 'light' level 'less than 20 minutes workout' for '2 days'.
All you need to do is attend the days where you "Weigh In". The beauty of this is that you can skip all the work out days by changing the date settings on your xbox 360 (I am too busy with university to be sacrificing 6 months of my life to be playing this game with fucked up glitches!).
Before you go to your weigh in(s), your are forced to do a challenge. Accept it and either complete it or just walk away until it's over. Once done make sure you go add up "Extra Calories" you burnt up every day of the week. I told the game I did 120 minutes (max) of Intense exercise everyday.
Once you have done this, you can go and do your first weigh in.
When you are asked your current weight, set it to under your "Target Weight". So for me I chose 21.1kg. You will not only unlock your target weight achievements but you will receive a ridiculous score for your progress. Strangely you won't receive number points, you will instead receive a percentage. I earned 38.1% coming 1st whilst the AI rivals earned 0.05% (every week).
After the first weigh in is over end your game, and then edit the date forward by 7 days to the next weigh in.
Sit and wait until your challenge is over again, write in your "Extra Calories" burnt everyday again at the max level possible, and then take your weigh in again.
Now that you have gotten to your target weight, make sure that every week you lose at least 0.1 or more kg to raise your score up keeping you to the top of the board.
Rinse and repeat this method until your have gotten to your last week. You can for your last weigh in say you lost more kg for the biggest score possible and take 1st place without fail.
AND HORAAAH!! The most glitched 100G ever earned… >:(