Go to a Forge tower, or ruins near campfires that have four floors (easier). If at a Forge tower, use Attract to bring enemies closer first. Stealth kill an enemy, and immediately climb back up and wait using stealth to check when you can stealth kill again. The enemies usually lose focus after short time, as long as there are not any archers that can spot you. "Height of Despair" will be earned after ten such kills.