You need:
2 people (same faction), one anything (DPS is best) and one priest level 30+.
What to do:
Find a humanoid you need to kill (for a quest, for loot, whatever), get near it (but don't aggro) and then start a duel. Once the duel has started, have the priest mind control the humanoid, then the other player can simply get to town on that mob, without either person having to take damage.
This works because you're dueling, and the mind controlled mob is basically considered a "pet" of the priest, allowing the other player to attack it. Without being in a duel the mind controlled mob would be friendly.