The key to defeating Scrapjack is very frequent use of the hammer. Deflecting rockets back at Scrapjack will drain his health very quickly, and almost eliminates the need for any special weapons. As such, keep a distance from Scrapjack and stay in the middle, deflecting as much of his rockets as possible. Be mindful of the obstacles in the boss arena and combine destroying those with deflecting projectiles where possible. A fast moving character such as Dancing Denzell also helps extra to avoid Scrapjack's projectiles.
For all details about the scrapjack visit the IHRB wiki page http://ihrb.wikia.com/wiki/Scrapjack