It's the final stretch! Make it through these rounds, and you're done with most of Billy:
Round 15: [7 sc, dec] x 4 (32)
Round 16: [dec, 2 sc] x 8 (24)
Round 17: [sc, dec] x 8 (16)
Round 18: dec x 8 (8)
Stuff as needed, and you're done! You might be looking at your piece and wondering – now wait a minute. Why is there a giant gaping hole in the middle of my piece?
Never fear, there's a tutorial here: