Once you get to the creeper sources on the last level, just put one blaster beside the creeper source. This will hold it off indefinitely (of course until the blaster goes underpowered).
Why the hell are ppl rating 1/5 just because it's CALLED a demo? If it wasn't called a demo, and just posted here with a link to the site that actually sold the full version, would you rate it 5/5?