20 days before the big day...Please help!

First thing: take a full-length test. You need to see how you'll do under real test conditions. Use that test to diagnose your strengths and weaknesses (and go beyond just "SC is a weakness" - which specific grammar rules are tripping you up? Then you can ask more targeted questions on the forum. It's too hard for someone to give you a strategy for SC in general - that's a 5-hour lesson, minimum!).

Generally, I recommend a test only once every 2-3 weeks, though people who are very worried about stamina or nerves may take them more often to build up stamina / try to reduce nerves.