I don't think that you even need a slider to be hard to hit. I think that if you have command of a decent fastball and a good changeup, you can be very effective. To me, the slider is a risky pitch. I never liked it and didn't teach it either.
I should've qualified my statement further... "hard to hit for me". I understand the mentality of "location" and "changing speed" to keep a hitter off balance (re: make them uncomfortable at the plate). But just personally speaking, a guy who just changed speeds or was throwing to corners wasn't hard to figure out for me. Not that I hit them hard, but on occasion I'd get some solid aluminum on them. Fastballs are the easiest pitch to hit for me even well located ones. Change of speed was not that hard because I tended to see the ball well and stayed on a pitch deep into the zone. I used a semi-slumped over the plate stance, bat comfortably on the shoulders and hands well placed and at the ready to flick the ball when needed, worked to get some solid swings too... very much a compact, stay level swing, no extra motion or hitch or timing motion for me.
The guys who really made me just shake my head trying to figure out how to hit them were the fastball/slider/changeup guys. If they spot their fastball outside, even if it's a showme fastball and not a strike, then follow with a slider or changeup, they're placing some serious notions in my head what to do with the next pitch. The slider was the hardest pitch for me to hit if a pitcher had me down in the count. If he threw a slider and I'm thinking fastball, I'm basically a weak arse swinging hitter who has no business being in the box.
I never threw a slider myself, went with fastball (what a joke, it was nothing anyone would call a fastball), changeup (had a nice one, but if they don't respect my fastball, the changeup is useless) and a nice curveball. And I got hit hard too, except for guys who were used to hitting a steady diet of fastballs (for some reason they were mystified by my changeup all the time)!