Once in the rear, you will find Guerra in one of two places: on the bottom floor underneath the balcony or beneath the canopy on the balcony. Keep an eye on your mini-map and look for the Boss's mugshot on your HUD to determine his location. If he is under the canopy, throw a couple grenades to get rid of his minions, then charge up and melee Guerra to death. If Guerra is on the bottom floor, be sure to take down as many of his thugs as you can before approaching. As such, descend the floors slowly while killing all of Jose's lackeys along the way. Use the staircases for cover or run up to a higher floor if you need to recuperate. Once on the bottom floor, you will find Guerra near a stack of boxes. Since he will likely be surrounded by other gang members, throw a grenade or two to clear most of them out, then finish your main target off.