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If none of the solutions work that you can see by the aspectRatio link in the JavaScript section in the Ninja Slider home page, you can consider the following approach. The aspect ratio is actually implemented by padding-top of the UL element in the slider. So you can force a desired proportion for mobile devices by the following CSS setting:
@media only screen and (max-width:800px) {
#ninja-slider ul {
padding-top:60%!important;
}
}