Scooter Gennett followed with a run-scoring single, and after Caleb Gindl walked to load the bases and Juan Francisco fanned, Schafer lined a 2-0 pitch into the right-center field alley for a three-bagger.
Johnny Hellweg (1-3) earned his first major league win, yielding three runs on four hits and three walks through six innings. He struck out one.
Rob Wooten and Brandon Kintzler each threw a scoreless inning and Jim Henderson notched his 23rd save as the Brewers evened their three-game series.
Arrieta (2-2) was touched for four runs on three hits in a five-inning effort.
Luis Valbuena clubbed a two-run homer and Brian Bogusevic added a solo shot for Chicago.