Physically as a Ronso Khimari is stronger and more agile, but we must remember that Tidus was able to take him on even footing at the beginning of FFX.
Balthier's had a good deal of training to him already and (and this is the clincher) is a gun specialist. Guns have a lot of stopping power, especially against stronger foes as they can penetrate armor and tough hides (Khimari having more of the latter than the former).
Balthier's quick enough to get a few shots off, which should do plenty to down Khimari. From the way it was portrayed, Ronso have an edge against humans in physical combat, but the gap is far from overwhelming and they don't possess an unnaturally high toughness.