Ugh. I had been trying the 7-builder way (if it's the one you're talking about, and I'm sure it is) for the longest time, and I got to the point with it where I just said "Screw this." I managed to trap them with I think it was maybe 7 or 8 (a very slightly different way), and it was sort of an accident when it worked. You actually can probably get away with 6 with this method.