Unlike Softened 'Em Up, for this trophy, you will want to go after and kill a racket boss without having killed any of their enforcers first.
You should just focus on all the other ways you can accumulate damage on the racket so that you don't need the enforcers. Some rackets only have one enforcer, so it won't be a problem to find one you can ignore.
Note: If you don't kill the enforcers before attacking the boss, they will be in the boss's location and you will probably have to kill them to get to the boss. This does not invalidate the trophy - you only miss out if you attack them in their spot on the map before the boss's mission.