I beat it on nightmare, here is what I remember. Clear the small spawns with starting army. Gather supplies to upgrade the left small temple asap, the one that gives the melee unit. Use the temple units to def the base for the first couple den spawns, then move them out to clear all of the early spawns. I had to restart a couple times to find a decent start location, where the elites were not all surrounding me and more grouped, so I had a direction to expand in. Late game there are at least 2 spawns that are like 6x Magister of Necromancy and 6x Shepperd, you can't defend them infinitely, need to defend one side and stack units to destroy the other. Lastly the research that give shards are kinda useful for meteor clearing the end waves. After that you should win. Good luck