For the achievement you need to accept the Scarlett Challenge 3 times, meaning you go for a Gigolo job. To open her challenge you must gather 1000 Guts points in challenges, each challenge give you 100/200/300/500 depending on the challenge. If you replay any challenge you gain 50 Guts.
As soon you get 1000 points, use it, 'cause if you accept the challenge with more than 1000 it will reset to 0 again.
To unlock challenges you can check my guide here:
Side note: When you complete your first Scarlett's challenge you won the "Cat eye's glasses" and you can use it on Gigolo missions to see through the clothes of the beauties.