Witch Time is engaged when you dodge an attack at the last moment. To engage it, you should dodge with
a little bit after you see something like their weapon flash. Time will slow down, making it easier to attack your opponents. Personally I found that dodging right when I saw the flash didn't activate witch time. I'd recommend dodging whatever attack comes at you just a small instant after the flash is seen. Once you get the timing right, you'll be able to activate Witch Time without too much difficulty. The achievement "You Want to Touch Me?" is cumulative. Once you've successfully engaged Witch Time 10 times, you'll unlock it.