Boy this achievement has come along way since launch eh
Ubisoft has changed this achievement to require getting 100 kills in casual/ranked multiplayer. This will come eventually and there's no fast way to earn this except to really learn how to play the game. If you want to increase the odds of you getting a kill, you can always hang out by the objective on defense.
Ubisoft has now unlocked all original 20 operators and attachments for all operators for everyone. This means that all you need to do now is load up the game and you'll unlock this achievement.
To unlock this achievement, you need to purchase 50 weapon attachments.
Weapon attachments can be purchased with renown, varying in price. Each weapon can have a variety of different attachments as well, such as sights, suppressors, grips and laser sights.
The best way I feel to do this is that once you unlock an operator, only purchase attachments that are under 100 renown. Each operator, depending on their weapons, has about 3-6 attachments that can be purchased for under 100 renown. If you start spending 200+ renown on one attachment, that adds up over time and you most likely could have purchased another operator plus the cheap attachments with them. (Not saying you can't buy sights and such, but this way is definitely the most cost effective way).