A spear trap is a section of floor that has spears which pop out at regular intervals. You need to get 20 creatures to die on these spears, either by knocking or leading them onto the spears.
A good place to get this is in Act 3. After the TV room, there will be a long passage with spear traps and several enemies. Kick or lead them onto the spears to kill them, and then kill yourself. Repeat until you unlock the achievement.
In Act 10-2 there are spear traps that materialize from purple vortices. These also count for this achievement, but there are very few enemies in the area.