I just beat the last level - I can share how I did it if you want to give it another go:

|| Rush upwards to block the enemy keeper from going for the initial gold SW of their base, This is a luck based attempt as the computer player has varied reaction times. After you pass the temple you veer right and then up at the guardpost. You need to possess an imp to rush claiming. Immediately drop imps to reinforce the walls where the computer will dig. You need to then possess an imp and claim all the neutral npcs on the map.

Claim the library and start researching with the Wizard and Monk immediately before you even build a hatchery, you need to RUSH workshop. Build a large hatchery to start getting bile demons. make a large training room. clear out a GIANT space for a workshop. I sold my initial lair and made my entire heart a workshop eventually. build a smaller workshop and dump your samarai/giant/thief in there. I would save the game at this point because you need to get lucky with attracting trolls and bile demons. once you have at least 8 bile demons and trolls (train them to level 2) use the steal hero and the 2 increase levels.

At this point I've expanded to the SW GEMS and moved my treasury down there. Expand your workshop to allow literally every creature who will work in there to do so. You are going for lava trap. use the lava traps to block off the access points where the enemy keeper would move into your areas the more the better 2-3 rows thick is what you are aiming for to start. There should be a chokepoint above the temple, one near your portal, and one leading down to your gold. Once the enemy claims that area, you FINALLY have some room to breathe, the enemy didn't bridge rush me until about 20 minutes later. The enemy will scavenge trolls, bile demons, and orcs, so you need to keep one of each in the temple. You should have the small prison claimed and torture chamber researched now. put as many doors as you can between your base and the enemy keeper. Their Imps will get stuck on the door. You can capture them and convert them.

Unlimited Imps for them is how you win the map. Your goal is to have the imps die and convert to level 10 ghosts. I had about 35 ghosts. There are waves of neutral mistresses, hellhounds, orcs, and bile demons, you want to covert all of these. make sure your ghosts as well as every available body are in your workshop, your imps don't mine the gems fast enough so you need to sell doors in order to heal. Once the enemy gets a few vampires they no longer scavenge anything but vampires, so you can now start to build a mistress, ghost, and bile demon army. I stole a few of the enemy keepers dragons as well.

At the final battle I sold traps until I had about 250k in gold. I had 16 bile demons, 8 mistresses, 35 ghosts, the horned reaper, 3 dragons, 8 orcs, the wizard, monk and samurai all level 10. I would save often along the way because your horned reaper or mistresses can randomly teleport into the enemy base and die instantly to a boulder trap. The final fight you need to be careful of positioning, make sure your call to arms is within range but FAR back, so the enemies get pulled into you, not the other way around. a few word of powers from the enemy dragons will instantly kill every ghost, you want them sniping from the backlines while your bile demons and reaper tank. You need to charge up a max cast heal and spam it repeatedly I ran out of gold, meaning I cast it almost 200 times.

Regroup here, dont rush the enemy keeper immediately because when they die, you have multiple groups of avatars and skeletons come from every angle, you can only take a minute or two though because you dont want the enemy keeper to re-amass an army. Once the avatars spawn, you need to kill them as they are approaching group by group, if you wait for them all to get your heart it will be too many. The avatars weren't that hard because I had the bile demons and mistresses the lightning mopped them up. One of the hardest maps I've ever played! ||