I think you just have to get used to the slide. After you do it enough, it will become a lot more comfortable. Keep your thumb flat on the back of the guitar. Like you are sliding it along a rod. And keep your thumb at the "fulcrum point" in the center of your fingers. Like, pretend that there is a pencil in your fingers. If you are holding the pencil with your pinky, pointer finger, and thumb, it should be gripped tightly and not twist or slip. This is the feeling you should get on the neck. I don't know if that made any sense.
I just realized that you can hammer on and pull off after making it to that Slipknot song on hard. There were some parts I was getting so frustrated with. I knew there was almost no way to "pick" those notes that fast.