I supplied & install materials but then couldn't pay my vendors or workers because the gc didn't pay me and excessively back-charged me for work that i didn't do because they didn't pay me
If you performed your last work less than 8 months ago (or 4 months for a single family home), then you are within your window to file a mechanic's lien.
How do I fight against back charges that are excessive and or not
How do I fight against back charges that are excessive and or not legitimate?
Typically, that fight is done in the context of the lien foreclosure action. The backcharges will be raised as defenses to the lien, and they will be fought over during the lawsuit.
How about this https://studymoose.com ?