Dungeons 3

Done in the "The End of Yaina Overproud" level. While it may seem like 75% is easy enough, it's a subtle bluff. The 2nd wave of Arcane Tears, each of the 3 Tears has a lot more health and does 500 damage to the Dungeonheart. During this 2nd wave you may have enough creatures powerful enough to take out all 3 Tears in under 60 seconds (to maintain We don't have time!) but in that 60 seconds the Tears would have caused more than 25% damage to the Dungeonheart.

In order to keep the Dungeonheart above 75%, you must survive the 1st wave of Arcane Tears, then assemble an army to the Overworld and destroy the Tears on the Overworld before the 2nd wave of Tears spawn, all in under 40 minutes total. Once you destroy the Tears on the surface, the Tears will stop spawning in your dungeon. It's also at the same location where Crowley is being held prisoner and is one of the level's objectives.

For this level's Overworld you can safely crush the Heroes with 10-15 Arachnids and 15 minutes is plenty of time to plough through the defenses to reach Crowley and the Tears, though you may have to return (via the Demon Gate spell) to deal with invaders 2-3 times.