Motionsports

There are 7 challenges on football and you need to diamond all of them to get this achievement.

1. Vertical challenge. This one is straightforward and easy. You'll be jumping and ducking obstacles. Over 3 waves, perfection is needed to get diamond.
2. Horizontal challenge. You will run 3 waves, dodging right or left or center. Again, this is easy, but perfection is needed to get diamond
3. Running back challenge. 9 tacklers will run at you from right and left, 2 in the 1st wave, 3 in the 2nd and 4 in the 3rd. To get diamond you have to use a move that is not explained in the tutorial: the spin. And you have to do it perfectly 9 times in a row. If you get touched or tackled, just start over; or use the 4 player choice at the start to give you 4 tries with minimal reloading screens.
To perform a spin: watch what direction the tackler is coming from, when the time slows down, move only the foot that is on the side of the tackler behind you so that your shoulder is facing the kinect. Also, make sure you lean away from the tackler or else he might still 'touch' you. You know you have done it right because the spin will give you 100 points times the multiplier.
4. Kicker. To get diamond you have to get 8 perfect field goals or 7 perfect and 2 good (out of 9). The 1st 3 chances are without wind and are quite easy- on all kicks do not kick too high, and on the 1st 3 kicks just kick directly forward and you should have 3 perfects (as long as you don't kick too hard). The next 3 have high wind, you'll have to kick into the wind, but again, not too high and not at a huge angle. These 3 are the hardest as there are 2 levels of wind it can be. The next 3 are light wind. Set up your kick just barely off center and into the wind and you should be able to kick another 3 field goals.
5. Quarterback challenge. If you follow the guide for the associated achievement, you will get diamond. 3 perfect throws and another 'normal' catch will get diamond. Just make sure you throw straight forward and anticipate the receiver's movements.
6. target challenge. This might be the hardest one- a score of 4000 is needed for diamond- in otherwords 8 out of 9 in the very center of the targets! Again, throw as straight as possible, practice a lot and it should all be about how hard you throw it. The 1st 3 throws need a light throw, but angle up to get the proper distance. The 2nd three need a slightly harder throw and the last 3 are slightly further than the 2nd. It will take a bunch of tries to get this, so be patient!
7. moving targets. This one is much easier, mainly because you need only about 2000 points to get diamond. The targets will be identical to #6 but are moving, so just wait until it is nearing stright forward and then throw. I got a bunch of 300s by accident and still managed to get diamond without too much effort.
Good luck!

These are the scores I got for my diamond medals so you have a ballpark to get your score around:

Vertical Run: 1700
ALL PERFECT

Lateral Run: 2875

Running Back: 2080
ALL PERFECT

Quarterback: 1650
3 Perfect, 1 Good

Kicker: 3900
6 Perfect, 3 Good

EDIT: Thanks to KDiamond1973
Standing Target Throw: 3600
8 Perfect, 1 Miss

EDIT: Thanks to getAtaxi
Moving Target Throw: 1250

Use the guide above to help you with each event. For Kicker and Standing Target Throw some say all you need is 3500.

Good Luck