Sans can't move his soul, which is why he should make it his mission to kill the human as quick as possible. The human has no invincibility frames and could die instantly from a single attack. But sans also dies in a single hit. Experiment around with the keys and see which attacks are which before playing. As for Frisk, use the arrows to move (down arrow to stab, go for sans' soul).
Thanks to Toby Fox for creating Undertale. I don't have the foggiest idea as to who made Ultra Sans, but thanks to whoever they are.
