Straight forward story mission. After you handled Remy and Olivia, you'd need to drive to French Ward to draw out Santangelo. Drive around the area until "retaliation" happens. Santangelo will chase you with his goons in a couple of cars. Kill them all to unlock this achievement.
An easy way to do these kill missions is to always call for your Hit Squad to back you up. They cost $3,000 a pop without markers - perfect for grinding out the $500,000 spent achievement.
You can also t-bone their cars, exit from the other side, and toss a frag grenade at them while they were still getting out. This would clear out a bunch of them rather effortlessly.
The achievement should unlock after the cut scene.