I unlocked this achievement by having 15 hats; 9 of which were unique. So, while I'm unable to confirm, I believe you can just load up one of the easier levels (Act 1, Episode 18) and just grab the same hat over and over again.
If you want all the unique hats though you can get them by entering the specific Act/Chapter even after you beat the game. You can also quit the map after you have grabbed the hat (without finishing the level) and it will count as being collected.
Here are the locations.
Hat #1: Act 1, Episode 8
Located in the bottom right-hand corner of the map.
Hat #2: Act 1, Episode 13
Located in the alleyway right above where you spawn.
Hat #3: Act 1, Episode 18
Under a sleeping bag towards the bottom of the map.
Hat #4: Act 1, Episode 21
Top right-hand corner of the map, behind the outhouse.
Hat #5: Act 1, Episode 24
Top left-hand corner of the map, next to a corpse right before the exit of the level.
Hat #6: Act 2, Episode 5
Bottom right-hand corner of the map, beside a well.
Hat #7: Act 3, Episode 7
In the building at the top-center of the map, in the room to the right of the innkeeper.
Hat #8: Act 3, Episode 11
Top-center of the map, behind the outhouse.
Hat #9: Act 3, Episode 15
Top right-hand corner of the map, after the well and wagon.
Hat #10: Act 4, Chapter 1
Top right-hand corner of the map, slightly before the exit.
Hat #11: Act 4, Chapter 6
Top left-hand corner of the map, in the house above where you spawn.
Hat #12: Act 4, Chapter 14
Bottom left-hand corner of the map, you can see it when you spawn.
Hat #13: Act 4, Chapter 20
In the second house on the right-hand side of the map moving north.