This achievement is for dealing anyone damage as a player demon. You don't have to damage the slayer, you can damage yourself or the minor demons running around to get credit towards this achievement. The Revenent demon can damage itself with its rocket attack, I'm not sure about any others. It's still time-consuming--a single
will only do about 10 damage. However, if you are boosting kills, you can sit and hurt yourself while waiting for the other guy to respawn.