For this achievement you need to wear 30 masks at once. The easiest way to achieve this is by earning 750 coins. You can get 750coins by doing missions or I got them by playing blackjack in the casino as I found it easy to win as it is more balanced to a player win than an NPC win. Once you have 750 coins go to any baa machine and buy 30 masks for 25 each then on the same screen you can equip all 30 to pop the achievement.
If you need any more help I will leave my video guide below.