OMD2 - The Hive - Nightmare - Untouchable - Manaless

Orcs Must Die! 2 - Nightmare - The Beyond Perfect Victory Guide

Alright, now we're talking business.

The Hive is the second map from the Are We There Yeti DLC, and one of the toughest maps in the game, if not THE toughest map.
Yet it can still be beaten solo on Nightmare mode, while getting the Untouchable and Manaless bonuses.

Here's one way of doing it.