Alvarez can be found on the beach behind his sporting complex inside a rocky alcove. While you can certainly attempt to get there via the main entrance, the many guards will not make it easy. A much easier 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 not have anywhere 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.