This achievement is earned by finishing a match with the 25 base game characters (Pendles, Ernest, Kid Ultra, and Alani don't matter)
Simply follow the self explanatory quests to unlock each character or reach rank 40 where you'll have have every character unlocked.
Update: as of this 17 GB update, every character should now be unlocked just by completing the Prologue
YOU CAN SEE IF YOU'VE PLAYED A CHARACTER BY GOING INTO THE CHARACTER SELECT SCREEN. CLICK ON A CHARACTER AND LOOK AT THE TOP LEFT: IF THEY HAVE 0%, THEN YOU'VE NEVER COMPLETED A GAME WITH THEM, BECAUSE IT GOES UP TO 3% JUST BY JUMPING OFF THE MAP ONCE WITHOUT KILLING ANYONE
Once you have every character, go to Private Story and choose 'The Saboteur' with Hardcore turned on. Go through each character jumping of the map at the start, the game will end and count as one game played.