I remember I had a bit of difficulty with this myself at first. I'm playing Heroic so most of the guards on the map were horde/throng/etc. A couple of things that might help:
1) The idea is to take everything on the map with minimal losses. The ONLY units you want to use are your Blood Furies and (in some fights) your Assassins. No other units (except maybe the witches but they're weak) can kill things without taking hits in return so don't waste your money on them.
2) In the first week, concentrate on getting your money buildings (ignore the resource silo), the Level 1 mage guild, and the Blood Fury building asap. With 5 Blood Furies and a Level 1 mage guild, you should be able to take 1/3 of the guarded stuff easy. Keep the assassins out of the big fights. If you're lucky, you'll have throng squires or some similarly speeded walkers guarding the gold mine (if it's a shooter or something really fast, you might wanna reload). I took that mine at the end of the first week with my 5 Blood Furies (that helps a LOT). If you can't take it by at least the 2nd week, you need to work on your skills with Blood Fury use (lots of hit and run).
3) In the second week, I built the hissing cavern/dark enclosure (I didn't use witches OR minotaurs in the end) and, more importantly, the castle. Your 10 total Blood Furies should be able to take at least another 1/3 of the map. The castle gave me 20 Blood Furies total by the third week. Don't upgrade any other building and don't recruit anything other than Blood Furies. With 20 of them, you should be able to clean out the rest of the map.
4) The very last thing I took on that map was the arena guarded by lots of imp. griffins. (If you had easier guards, you might wanna take that sooner).
5) Don't bother with the elemental chains building on this map as you won't have it for the fight at the end.
6) Make sure you get Adv. Destructive magic and build up to mage guild level 4 (your hero should cap before you head down). This netted me empowered meteor shower, which makes a huge difference. You can build the mage guild on the last few days.
7) Upgrade the Assassin place (forgot name)/Hissing Cavern/Dark Enclosure during the last week.
If done correctly, you should have enough money for ALL the creatures in the stockpile (assassins, furies, grim, deep hydras) with money to spare.
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