If this exercise is too challenging, you can build up your core by doing the alternating dumbbell chest press on an decline bench. (all-about-arm-exercises.com)
If this exercise is too challenging, you can build up your core by doing the alternating dumbbell chest press on a flat bench. (all-about-arm-exercises.com)
This page shows you pictures of the 1 arm dumbbell chest press on a stability ball. (all-about-arm-exercises.com)