Ele nunca morreu, apenas sua forma física foi destruída durante aquela batalha.
Então a sua pergunta seria: Quem derrotou Sauron na Guerra da Última Aliança? A resposta certa seria Isildur .
Como retirado do link , que é totalmente preciso e resume a batalha muito bem:
Then Gil-galad and Elendil passed into Mordor and encompassed the stronghold of Sauron; and they laid siege to it for seven years, and suffered grievous loss by fire and by the darts and bolts of the Enemy, and Sauron sent many sorties against them. There in the valley of Gorgoroth Anárion son of Elendil was slain, and many others. The helm of Anárion was crushed by the stone-cast from Barad-dûr that slew him.
But at the last the siege was so strait that Sauron himself came forth; and he wrestled with Gil-galad and Elendil. Elrond beheld the last combat on the slopes of Orodruin […]. Isildur alone stood by his father in that last mortal contest; and by Gil-galad only Círdan stood, and Elrond. With the heat of Sauron’s hand, which was black and yet burned like fire, Gil-galad was destroyed. But Sauron also was thrown down, and with the hilt-shard of Narsil Isildur cut the Ruling Ring from the hand of Sauron and took it for his own. Then Sauron was for that time vanquished, and he forsook his body, and his spirit fled far away and hid in waste places; and he took no visible shape again for many long years.
A maioria dos contos da Guerra da Última Aliança está espalhada pelos livros Senhor dos Anéis (citações encontradas na resposta de Edlothiad) e História da Terra-média: O Retorno da Sombra .
Isso responderia ao seu segundo (uma conta detalhada) também.
Para sua terceira pergunta (local da morte de Elendil e Gil-galad): Naquela época eles estavam participando do cerco de Barad-dur; então é provável que eles tenham morrido antes do Barad-dur enquanto estavam em combate com Sauron.