This requires you to complete all paths of the Carnales section of the missions.
That includes all MISSIONS labelled with the Carnales icon, and all STRONGHOLDS with the same. Having played through these very recently, I can assure you they're all very straightforward and more time-consuming than challenging. Use squad-members to your advantage and remember to restock on ammo before a mission.
Once you've completed all of these (I believe there's 14 in total, so you'll need a 14X multiplier of respect), you will gain access to the final mission.
The final mission is a bit of a pain involving stopping a plane from taking off whilst being chased and shot at by RPGs.
My advice for this level is to use RPGs to your advantage and blow up the cars chasing you ASAP. Fire a few off at the plane every now and then, use the AK to take out RPG-launchers (you have unlimited ammo but switching between the two is imperative). When the RPGs are gone, you'll likely sail through the rest of it, leading you very close to the plane and allowing you to finish it off with 3/4 RPGs.
It can get very frustrating and you rely on luck a lot of the time, but as long as you have a feel for the route and remember to switch between weapons, you'll be fine.
(Upon unlocking this you also unlock a new 'crib' and a selection of cars)