The 300 level at Kauffman Stadium is your best chance at beating the sun and the heat. The Loge seats, as they're called, are covered by the 400 level overhang, which provides the best protection anywhere in the ballpark. Given their convenience and cover, these seats often sell for more than lower level tickets.
Additionally, there are fewer rows in these sections, which means less walking up and down stairs to your seats. Sections 301-311 on the third base side are the preferred option as the sun will set behind them during late afternoon and evening games.