You need to match a total of 10 police tokens in one fight and go on to win. These are the bad symbols which you normally avoid matching because they damage you.
This can be done fairly easily on the very first fight of the game, as this is a tutorial battle the AI does low damage every few turns. If necessary, you can restart this until you get a starting board with many police tokens (press
to quit out to the main menu).
You will need at least 2 match-3 and 1 match-4 lines of these tokens and then as soon as you win the fight the achievement will pop.