With permission from runner Hodgey:

The primary trick in this level is to lure heroes away from their dungeon heart by building and selling bridges across the lava. Build the bridge, they path in, sell the bridge, they go back to wandering aimlessly (a similar trick works with guard posts next to water). Only flying heroes then remain a problem: one fairy and two priestesses which the claimable bile demon can handle. The lord of the land can (with a fair amount of luck) be killed with one of the two boulder traps near the heart.

This was in no way a good run: I fumbled and panicked and misclicked, but also I haven't got the timing down for when to place the rooms etc.. There is a certain amount of luck in how the heroes path over the bridges, and a lot in how the final group is placed (thus whether the boulders catch the LotL). Nevertheless, I think the strategy is generally optimal: get to the heart, moving the heroes out of the way rather than fighting them when possible, and kill the LotL with a boulder.

You might notice this is being uploaded out of order: I'm routing them as I go, and this was fairly low-hanging fruit. I think for several levels there will be no alternative but to actually attract and train a creature...