Dungeons 4: Doom Hill on "Extremely difficult" in 34:43
Doom Hill is one of maybe three levels in the campaign I consider actually genuinely difficult on "Extremely difficult", and that's because it's an Undead level!
While you don't absolutely have to go undead, you do start with a massive investment into undead, and units which aren't undead will take heat damage, so you have to choose between babysitting crappy undead units, or babysitting superior horde/demon units so they don't swiftly die in a fire, demons especially die very fast as they don't have naga healing.
I actually tried it with demon, horde and undead, and ultimately concluded undead is the least painful faction to focus on despite being undead.
So you may as well use the Bad to the Bone perk to enhance your undead. As on other undead levels, toss gold or mana in the sacrificial pit to get fast blight so you can have minions for damage and tanking without needing to waste time generating blight in the slower ways. Then just plod on through the level, trying to find opportunities to move the stone fragments.
I think it would've been a bit easier if I got Horde Strength instead of investing in undead unit upgrades, because +100% damage is basically better.