Arroyo (9-7) logged his 100th victory as a Red in the 108-pitch effort, during which he didn't issue a walk and struck out six.
Todd Frazier knocked in three runs, while Devin Mesoraco (3-for-5, two RBI), Jay Bruce (two RBI) and Shin-Soo Choo each socked a solo homer, leading Cincinnati's offensive onslaught.
Derrick Robinson went 3-for-5 with three runs scored in the victory, which opened a three-game series.
Giants starter Tim Lincecum (5-10) was torched for eight runs on nine hits over just 3 2/3 innings.
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