This achievement is quite simple when it comes to the hidden ones in the game, but takes a while and has a bit of set-up needed to get to it.
Start off by following the "A Close Shave" Mission Story. This will help you identify the Maelstrom (which you could kill if you save at a certain point later) and get you into the Barber's disguise. You will need to go into the Crow Hideout to find a picture of The Maelstrom, then go to the Barber shop and get his disguise so you can open the shop up.
That's the small set-up done. Now begin serving customers (use instinct if you see a purple objective marker on the map, as these are possible Maelstrom's), get them in the chair and end the shave as soon as the option pops up. As soon as you discover the Maelstrom, serve him straight away (and DON'T kill him, otherwise he doesn't count as having been shaved), as he never comes back and is needed for this too (save here if you want to get the kill on him in the Barbers chair during his shave).
Continue shaving people, there will be 15 in total including The Maelstrom (you may have to wait around at 13 or 14 done for someone to show up at the stall for shave). Once you hit 15 people, you'll get a challenge and the achievement, again, a simple one that just takes a bit of time is all.