how to fight against a mechanic's lien?

I am a homeowner who hired a GC. The initial bid on a home improvement project (siding) was for 50% down and 50% upon completion. Extra work had to be done and the GC demanded immediate payment (with no prior notice of such a demand) of $5,000 on the extra work on a Friday, and then on Monday, when I gave him the $5,000, he demanded immediate payment of the remainder $3500 of the extra work (because it was all done) and an additional $7,000 on the work from the bid that had been completed up to that point. When I pointed out that the extra work was not all done and his original bid was for 50% upon completion, he pull his workers. He left me with no siding and only some trim work done. I had to hire another crew to get the siding put up on my house. Now he has lawyered up and is demanding some other number that was never discussed, plus a mechanic's lien threat.