The Volk I vanquished Thursday. Only the Shai-Gen remain. Not all of Pacific City's three boroughs have a final boss, though they each have a final crime. Los Muertos have a final boss, the Volk have something different.
I found the Volk's final crime easiest to tackle with a vehicle. Preferably one with guns, like the max-level Agency supercar.
Friday I killed the first of the Shai-Gen generals, after spending some time in Los Muertos territory grinding my agility skill. I'm starting to really like the rocket launcher - it fires like a gun, but gives really satisfying explosions and boosts the explosives skill.