Xoff submitted this

If you need to built a bridge across water or lava, send high level warlocks along to guard your imps. They'll blast any enemies they see (especially archers) while your imps build bridges.

Build a medium length (4-5 tile) corridor and place locked doors at either end. Fill it with poison gas traps and 2-3 bile demons. Use imps or other small creatures to lure heroes to the area, then unlock a door and lead them in. Lock the door and the enemy will be gassed by the traps AND get beaten by the demons, who are immune to the gas.

More advice on the DK assistant?