Head to the Broken Shillelagh to start this quest. This quest is divided in several section. You'll start in your simulated crib. Help Pierce take out the Saints Flow mascots. Next, find some guns. Follow the markers until you collect them all. Finally, you have to pick up the RPG from the helipad. Go grab it and use it against Paul, the monstrous Saints Flow mascot. Keep firing rockets at him and pick up more rockets from an ammo cache nearby. Keep doing this until you get thrown to a far off building.
Next, you have to fire rockets at Paul again. If you don't have any ammo, wait a bit, preferably behind some cover and wait for him to throw debris at you. There should be an ammo cache along with it. You'll also need to deal with some annoying cans. Kill them all and just repeat the process until Paul falls.
Ride the helicopter next. And keep firing at Paul to get its attention. Keep doing this until you land near the island statue. Enter the statue to take control of it and use it to deliver crushing blows to the monster can. Keep at it until a Y appears over it. Press the button to land the finishing blow. The quest will be completed shortly after.
Pierce will now be available as a crewmate and homie. Two new sidequests will become available as well.