Portland (16-10, 7-7 West Coast) held a 10-point lead with 2:42 left, but LMU's Evan Payne wasn't ready to concede anything to the Pilots.
Payne - who scored 21 of his 23 in the final 6:27 - scored 11 of the game's next 17 points to pull the Lions (8-19, 4-11) within 61-58 with 21 seconds to play.
After Portland's Alec Wintering hit a pair of free throws, Payne drew a foul on Pressley - but made just 1 of 2 from the line.
Two free throws by Gerun pushed the Portland lead to 65-59 with 11 seconds left.
After a missed 3-pointer, Payne's offensive rebound and putback made it a four-point game with three seconds to go. Portland turned it over, Payne was fouled and - after making the first two free throws - missed the third intentionally.
Jason Todd secured the rebound, and the win, for Portland.