This achievement will come naturally as you play through the game and progress through the levels. Your characters level up during the game and you will be notified of this. I would suggest playing with friends or online with random members of the game as it makes the levels much easier to progress. One tip at the moment is that the game has a glitch where every character is levelling up very quickly and at the same pace (no matter which you use more). This achievement is tracked and will be unlocked through natural progression (most likely after the first story board). Overall, if you pick the same characters from the initial first level they should level up at the same time and the achievement should unlock quickly after they all reach the target level.