This achievement can be obtained in player matches as well as ranked matches. To get this achievement, you have to WIN 50 rounds of a merc. To WIN as a merc you have to escape the heist with the most money. To escape with the most money, you either have to be the only one to escape, or escape as a traitor with the most money. I know a merc is a traitor, but just to explain it better, I will class a merc as someone who escapes with their gamertag white. So, if two mercs escape in a round, they split the money between them and therefore none of them win as they both have the same money. To WIN a round as a merc, you have to be the only person that has their gamertag white.
To WIN as a traitor, you need to have the most money after you have escaped. A traitor doesn't split their money with the merc they escaped with so for example if a traitor escapes with $1million and a merc (white gamertag) escapes with $800 thousand, the traitor would win as they escaped with the most money and doesn't split it. So to recap the achievement, you can only win as a merc if you escape with the most money by being the only merc to escape or having the most money when escaping as a traitor. Hope this guide helps for this confusing (and fustrating!) achievement.