Once in the rear, you will find Guerra in one of two places: beneath the canopy on the balcony or on the bottom floor underneath the balcony. Keep an eye on your mini-map and look for the Boss's mugshot on your HUD to determine where he is. If he is under the canopy, throw a few grenades to get rid of his minions, then charge up and melee Guerra to death. If Guerra is on the bottom floor, it is in your best interest to take down as many of his thugs as you can before approaching. As such, descend the floors slowly, 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 go in for the kill.