I tried soloing it with three templates: standard tamer/mage with GD (lasted like ten seconds before being teleported and killed); standard sampire (managed to hit him a few times, then I got discoed, peaced, explosioned and something else at once, and decided to leave); my necro disco archer. This one fared a little better - I managed to disco the Harbinger and hit him a few times, but as soon as he discoed me back, there was no way to keep on fighting, and whenever I left he healed himself, so...
Finding good karma creatures in a secluded spot is the easiest way by far, I think.