Alvarez is found on the beach behind his sporting complex, inside a rocky alcove. While you try to get there via the main entrance, the many guards will make it difficult. A safer method is to enter the ocean from a nearby beach and swim around to the back of the sporting complex. When you reach the shoreline, run up the beach, take out the enemies ahead, and jump on the rocky structure where Alvarez waits on the other side. From this vantage point, Alvarez will have nowhere to hide. Toss grenades to not only damage him but also clear out his henchmen. When it is safe, drop down and finish Alvarez off with melee attacks.