This achievement is linked to your characters Badass Rank. These ranks are gained by completing challenges given to you throughout the game. Upon completing one, you'll be given a new Badass Token which you can then put towards a new Badass Rank.
There's no limit on how many Tokens you can receive and Badass Ranks you can gain, so all you need to do is obtain and redeem 25 Tokens. To use a Token or see how close you are to the next Badass Rank, open your ECHO Device and tab across to the Badass Rank/Challenges page (Medal icon).