The senior guard fired 18 points on 4-of-8 shooting from deep, to go with seven rebounds, four assists, and one steal.
Altamirano blocked Nat Cosejo's potential game-tying drive as the Golden Stags escaped with the victory.
He drew an unsportsmanlike foul from Cosejo but failed to ice the game as he missed both of his freebies and still needed Jessie Sumoda's split to stretch the lead to three in the last 2.8 ticks.
Romel Calahat had 12 points, 10 boards, and four assists, as Ameer Aguilar had 11 points on 3-of-9 clip from beyond the arc, as well as three rebounds in the San Sebastian win.
This Golden Stags victory pushed their record to 8-9, while keeping the hosts Generals at the cellar at 2-15.
Cosejo paced EAC with 15 points, as Ruzzelle Dominguez had 12 points and five rebounds in the defeat.
San Sebastian 62 - Altamirano 18, Calahat 12, Aguilar 11, Sumoda 6, Cosari 4, Villapando 3, Una 3, Escobido 2, Are 0, Concha 0.
EAC 59 - Cosejo 15, Dominguez 12, Tolentino 7, Maguliano 6, Cosa 6, Quinal 5, Balowa 4, Luciano 2, Angeles 2, Bajon 0, Umpad 0, An. Doria 0.
Quarterscores: 20-15; 39-32; 53-44; 62-59.