This one is Vengeance- Exacted Revenge on Julius.
To get this, first go to the Stilwater Police Deparment in the southern area of the Saints Row district. This is the same building as the next to last Sons of Samedi mission where you hijack the traffic camera feeds. Enter the building and take the stairs up to the third floor and in the southwestern corner there's a door to the Detective department. When you enter, you should see three Saints triggers for three wiretap conversations. Listen to all three, then go into the adjoining room (Troy's office) and find a fourth, where you'll get Dex's number.
Call the number and he'll set up a meeting at the Saints Row church. When you get there, you'll meet with Julius, but it's a trap. Fight off three waves of Ultor troops in the church, then four waves outside followed by an attack chopper. After this you'll go to Julius's car and ride shotgun towards the Museum ruins. You'll have infinite ammo of all weapons, and simply use the rocket launcher to destroy and troops trailing you. Julius will crash in the roman amphitheater surrounded by more Ultor troops, but your task'll be to shoot down the two attack choppers. Do so and watch the cutscene to complete the mission and unlock the achievement.