1st: Select an enemy building
2nd: Press ctrl+d simultaneously
3rd: Watch the building turn instantly into dust! This was fixed in the Battle Pack 2 (1.10j) version of the game.
Quickly build a Tomb of the Brothers and summon Lythis, one of the brothers of the crypt. Just have him attack a tree and watch yin points add up. This trick works well on a stage with lots of trees. (Note: This was fixed in later patches)