Cruel Smash Guide

So here is a guide if you have problems with Cruel Smash. First, choose Lucina and hang on the right side of the edge. When the Miis come, jump down and use your Up+B to grind up on the side of the stage. This way, the sword clips through the stage and hits the Miis. After some time, the Miis might fall down, just keep on trying! Here is also a video from GameXPlain showing this trick.