Stauffer (8-9) allowed one run — John Buck's 15th homer in the seventh inning — and five hits. The right-hander struck out five and walked one in his first outing since he yielded five homers and nine runs in just three innings of a 13-1 loss at Cincinnati.
Florida starter Javier Vasquez (7-11) allowed three runs and five hits in six innings. The right-hander retired his last 14 batters but still dropped to 0-2 with a 2.77 ERA in four starts this month.
Florida has lost 12 of 14 to fall a season-high 24 games behind NL East-leading Philadelphia.
San Diego scored twice in the first inning, but missed out on a chance for a bigger inning.
Jason Bartlett hit a one-out double and Cameron Maybin reached on an infield single. Guzman followed with a two-run triple. Orlando Hudson then was hit by a pitch and stole second base with Kyle Blanks at the plate.
Blanks stuck out on Hudson's steal of second. When Buck's throw caromed off the glove of second baseman Jose Lopez and went into center field, Guzman came home. But plate umpire Tony Randazzo ruled Blanks interfered with Buck's throw on his swing for the third out of the inning.
Nick Hundley led off the San Diego second with his third triple in as many games and scored on Logan Forsythe's sacrifice fly. Hundley has hit safely in all six games he's played since returning from the disabled list, hitting .524 (11 for 21) with a homer and three RBIs.
Florida's Mike Stanton went 1 for 2 with two walks, but failed to homer for the first time in five games.
NOTES: Marlins manager Jack McKeon was back in San Diego for the first time since he managed Florida in 2005. McKeon was the Padres' general manager from 1980-90, and served as the manager from 1988 to 1990. ... RHP Joe Ross, San Diego's first-round draft pick, threw out a ceremonial first pitch to C Austin Hedges, the club's second-round selection. Hedges caught a bullpen session before the game from LHP Cory Luebke, then took batting practice. ... Marlins RHP Chris Volstad (5-9, 5.60 ERA) will make his second start since being recalled from Triple-A New Orleans on Friday night. He will face San Diego LHP Wade LeBlanc (1-2, 4.54), who has not allowed more than three earned runs in any of his six starts this season.