Gorzow breathed life into their campaign with a 79-72 triumph.
Both sides are now level at 2-5 in Group C and chasing the fourth-place finish required for a spot in the Eighth-Final Play-offs.
Wednesday's game was knotted at 67-67 with 3:45 to go when Spencer connected from long range.
After Sopron's Tijana Krivacevic hit a jump shot to reduce the arrears to 71-69, Spencer made her second big shot from behind the arc for a 74-69 advantage just 2:20 from the end.
Sopron's Amber Holt split a pair of free-throws to close the gap to four but Gorzow had the wind in their sails and went on to score the next five points.
Samantha Richards made a lay-up for a 76-70 lead and after two missed shots and two turnovers by Sopron, Justyna Zurowska buried a three-ball with 14 seconds to go for a 79-70 lead.
Maja Miljkovic sank two free-throws at the end for Sopron, two points that were too little, too late.
Liudmila Sapova poured in 20 points, Spencer 15 and Richards 12 for Gorzow, who forced Sopron to turn the ball over 25 times in the game.
Sopron, who reached the Final Four last season, got 14 points from Katalin Honti.
Norbert Szokely, coach MKB Euroleasing:
"This was very difficult game, because this was basically our last chance of securing promotion to Top-16. In the first minutes of the game we played well in defense and used our pace in offense. But Gorzów later in the game began playing better in defence and they won. My team could not show what they can do, we lacked speed."
Dariusz Maciejewski, coach KSSSE AZS-PWSZ Gorzow:
"Once again, we won the game in the fourth quarter. This means that we are prepared very well for the season. We were a team today and that's why we won."
Katarzyna Dźwigalska, player KSSSE AZS-PWSZ Gorzow:
"We had several hard games on EuroLeague level, and when we won we did because of our improved performance in the last part of the game."