All you have to do is level up in a four player game as the achievement states. The easiest way to achieve this is get 1 more person and both use a second controller to load up a guest. Let them both idle out (takes about 30 seconds) and the computer will take over for them but they will still count as players. Then all you have to do is level up once and the achievement should pop.
If you want to make it even easier just start a new game and after you talk to Odo for the first time you should automatically level up all 4 characters.