The slicer is a large spinning blade that slices across a room, either from the wall or floor. You need to get 20 creatures to die on these blades, either by knocking or leading them onto the blades.
While there are many locations with slicers, the best place to work on the achievement is in Act 3. You'll come to a point where there is a locked door at the end of a large room, and you need to pull a lever to open the door. Pulling this lever also activates a pair of large slicers, and lets lots of creatures into the room. Simply bait the creatures into walking into the blades, and they will die. Once the room is clear, kill yourself on the blades, and repeat until the achievement unlocks.