Seven Bullets Zombie Apocalypse

You have to buy the most expensive hat for gaining this achievement (Costs 2500 coins). Don't waste your money on the other hats (like I did since I thought I had to buy them all) and instead, upgrade your bullet capacity which will help you to achieve other achievements faster.

According to my experience, it is not dropping from regular big zombies. There are specific ones with mustache and beard. You have to encounter one of them and kill to get this drop. Also I am pretty sure, chances them to appear with mustache and beard is much less than 20% as DEV mentioned in the game's forum. I could say, it was not more than 5% (I have encountered just 2 or 3 of them and 50+ regular ones) for me during my whole playthrough.