A spiked wall is a section of wall that has spikes protruding from it. You need to get 20 creatures to die on these spikes, by kicking or punching them into the spikes.
In Act 3, after the passing through the passageway that has lots of puddles, you will turn the power on which causes the puddles to become electrified. At this point you need to go down a ladder to proceed. In this underground passage, there are spiked walls and a handful of enemies. Kick or punch them into the spikes to kill them, and then kill yourself. Repeat until you unlock the achievement.