Garrett Danner had 12 saves, but the Bruins' offense scored the fewest goals in six years.
Senior Daniel Lenhart led the Bruins with a hat trick against the Gauchos.
The Bruins will face the winner of No. 8 Pacific/No. 9 UCSB on Friday at 3:00 p.m. at Long Beach St.
Gordon Marshall scored four goals in the 16-8 win over No. 4 Long Beach State.
Eight seniors played in their final home match, but sophomore Gordon Marshall led with four goals.