The Emissary of Dhuum is a short trip northeast of the Gate of the Nightfallen Lands. He appears to be alone and an easy target, but don't be fooled. If you approach to melee range, large groups of demons appear. Fortunately they are closely grouped and so AoE attacks work well. Use party-wide protection spells and chants and have fighters keep the groups of enemies penned in and away from the caster backline. Note, you only have to kill the Emissary to complete the quest, so the entire party can focus on him to quickly take him down and map out without having to fight the other spawns.