When you complete challenges you will get badass rank and after a certain amount of badass rank you will get a token which you can redeem for bonus stats. The amount of badass rank you receive depends on the level of the challenge. You get 1, 5, 10, 50 and 100 badass rank for completing respectively a lvl 1, 2, 3, 4 and 5 challenge. Meaning if you complete a challenge from lvl 1 to lvl 5 you'll get a grand total of 166 badass rank.
The amount of badass rank needed for a token increases with each new token and to obtain 25 tokens you will need a badass rank of 325 and that will come very fast and easy. You will likely redeem several hundred tokens throughout your quest for the achievement.