As usual it falls to you to sort it out, dragging a couple of your own gang members in tow to help.
Head over to the Front Page bar and take out the guards protecting it to find out who is supplying them. Take the Sentinel conveniently parked outside the mansion so you can fit all three of you in.
Just run the guards over or get them with a drive-by and enter the purple indicator. The owner tells you where the rival security firm operates from and you have five minutes to deal with them.
It’s handy to have some grenades at your disposal as you can just throw them over the wall to deal with the first set of guards. Alternatively you can park a vehicle in front of the entrance to block the guards in and then explode it with grenades or a rocket launcher.
It’s best to keep a fast car nearby as you’ll need it for the next part of the mission when two more guards on bikes escape leaving you to chase them down.
They ride off down the main north-south road on the east of the island and if you have a reasonably fast car then you’ll just need to ride alongside and drive-by or knock them off. What’s more they’ll have M4 ammo for you to take as a bonus for completing the mission.