In order to be able to dodge you will first need to charge your Ki Meter to 25%. There are two ways to do this, one with QR codes and one without.
Non-QR Code Method:
Make sure your difficulty is set to easy and that you are fighting Raditz (Score Attack > Saiyan Saga > Raditz). Pick any character (preferable one with more health) then charge up your Ki Meter until you get the option to jump (located above the Ki Meter), once you have jumped and done the Assault Gauge thing, immediately begin charging your Ki Meter. I would suggest filling it as much as you can rather than just 25%. From here just avoid taking as much damage as possible while not attacking, and eventually (possibly after some time) Raditz will launch his Super Attack, at this point lean to your left and it should dodge. Just repeat this process and you should get it eventually.
QR Code Method:
You have two choices here, you can use the QR code for Unleash Ki, which will give you a maxed out Ki Meter permanently or Eternal Youth, which will mean your health never goes down. If you use Unleash Ki, you might be best to turn the difficulty up, which should hopefully increase how often Raditz will use his super attack, although you are more likely to die quickly, so if that is happening just adjust the difficulty to suit. Regardless of difficulty it is basically just a matter of waiting for him to use it. For the Eternal Youth method, just turn the difficulty to easy and charge your Ki Meter as often as you can (possibly refer to the Non-QR Method on getting some distance) and wait until he uses his Super Attack. As long as you never attack Raditz, and fail everything except dodging his Super Attack, the battle will never finish, so just go until you get the achievement.