This is one of three missions available to you at one time. Note that after finishing two of these, the simulation will be overrun by a lot of gang members, so it would be wise to do the A Game of Clones to get rid of them.
Head to the usual location and enter Pierce’s simulation, where you will find him facing off against Saints Flow mascots. Help him out and kill them until you hear a scream, which prompts Pierce to tell you to get more guns. The SMG is upstairs and the shotgun is after that, although you’ll see a "tiny" distraction when grabbing that one.
Ignore the distraction and keep collecting guns, with the next one being a rifle and the last one being a RPG. When you pick up the last one, a giant Saints Flow mascot appears, whose name appears to be Paul for some odd reason. Stay where you picked up the RPG and fire at Paul, only moving from the position when you need more ammo.
Once you’ve hit Paul enough, a cutscene will occur and then you’ll be alone with it. Paul will be throwing debris at you and throughout this portion of the battle, smaller versions of the mascots (like when you first entered the simulation) will populate the arena. As before, fire at Paul with the RPG, then kill the smaller mascots when they appear, only stopping when you need more ammo.
When Paul falls down, Kinzie will chime in and say she knows how to defeat Paul for good. At this time, Pierce will pick you up in a helicopter and start flying to the destination. While flying, you’ll have to fire at Paul to keep his attention on you. Once you land on the island, head to the large statue and you’ll get inside. While there’s a button for attacking and blocking, all you ever really need to do is attack, so keep doing that until a button appears over Paul. Press it and the battle/mission will come to an end.