This achievement like the Orc and Grunt Slayer can be done while AFK. Load up the level Demon Pen on hard mode with the Wizard(Elf could work as well).
Kill the few enemies that come at you and move directly to the top of the map. Do not lower the bottom bridge. Lower the top bridge and immediately after hit "W" on your keyboard or up on the gamepad. You will notice you character move up a step into the crank for the bridge. For whatever reason if you stand there the Demons will pile up but will never attack you. Using the Wizard, Spam the Ice beam and find something to keep the button pushed down (The Elf's arrows should work for this as well in theory). You will now get kills while AFK.
There is one hitch though to this solution. It is recommended you are nearby and checking the game every 10 - 20 mins. The spawners tend to spawn the flying demons which normally stay to the right out of harms way but they occasionally fly over and do kill you. In which case you need to respawn kill the stupid thing and get back into position. It should not be a big deal as I had always gained plenty of skull coins before dying. It doesnt happen too frequently. I easily gained 8 skull coins doing this. This method can also help you achieve the 8 deaths in 1 level.