If you don't get this during normal playthrough (I was about a quarter way when I finished as I didn't pay much attention to the style I was using) then it's easy enough to pick up post game through level select.
Pick the level The Unforgiven: C-Town (just labelled C-Town on the map). The chapter starts off in the bar, and providing you don't destroy the sound system you will get infinite enemies.
To master slice enemies you need to select Master Style by pressing
and then getting a 5 hit combo (with all skill options maxed out - you should have more than enough skill points just playing through the story) with
and then hitting
for a finisher. It'll say on the right of the screen when you're able to pull it off.
Now just repeat until you get to one hundred. You can check your progress anytime on the pause menu. Don't worry if you die, as the slice counter is cumulative.
There is one point where you will have to switch to Afro style (
) to deflect about 5 daggers before more enemies will spawn, but mostly it's just avoiding the electricity (which renders you temporarily style-less) and putting up with the constant shouts of "Oh-oh - ninjas in the hizouse!" which can get a bit grating.