Can't you keep combos going past 0 health, then once you drop it or end it they enter the Finish Him state? If that's the case I would assume that you just end the combo with an iceball for the brutality.
To perform his EX Ice Ball Brutality, the clock has to have more than 40 seconds and when your opponent has a small pixel of health left, you have to freeze them. While they're frozen, you need to be fullscreen distance then perform the EX Ice Ball and it will cause a Brutality.
Many brutalities are hidden in chests in the krypt which show up in certain place at certain time, like it was in mk deception so you have to roam around and get lucky to be in the right place and right time to find the chest.