Bronson Arroyo allowed 89 home runs since the start of the 2008 season, the most in the majors. His pitching against lefties demonstrates why. The following graph shows his pitch location and movement against lefties over the last three seasons:
The power against Bronson happens when he comes too far inside, leaving the ball over the middle of the plate. Note to that the hot spot for movement, where batters get the most power, is located between the major and minor hot zone on the graph above. So Bronson does well when his ball stays fairly straight, or when it breaks in on lefties a lot. When that break is in between, the ball stays over the middle of the plate, and batters hammer it.