Ok...this will take some practice for some of you but others will be able to unlock this on their first try - especially if you have played any of the Madden mini-games.
First, change your controller setup to the alternate version (where A is juke and B is spin). A lot of gamers have reported success with this setup. Then select Tackle Alley from the menu.
Try to attack the first defender at an angle. See my generic diagram below (X is defender, \ is your running angle)
Then spin or juke as the defender commits to the tackle. This will take some practice to get the timing down. Then be ready to spin or juke for the next defender - again, trying to attack the defender at an angle.
Also, the Waves never change. For instance, Wave 19 will always be the same - X amount of defenders in a specific defensive scheme. So, take notes. If you know you used juke for Wave 8 and only juke, then write it down. You will need as many lives as you can get for when you try to do all 100 Waves.