Our strategy for Wreck the Halls level 18 is lob the White bird into the gap in the middle of the structure. With good aim and timing, the whole left-side of the structure should collapse and at most 2 pigs should remain. Use the first Yellow bird to reach the far-right pig, trying to dislodge the boulders for some extra damage points. Use a high-arcing shot with the last Yellow bird to get the pig on the left edge. The score in the video below is 71,460.