Just keep sticking with it. It'll pay off on test day when you get a bizarre passage.
The key though is to not freak out. Many complex passages contain extra information, or may seem advanced, but are still really only testing a basic concept. Stay calm, and just focus on: what basic science knowledge is this passage asking me or getting at? Take a deep breath and dive back in if necessary. Sometimes, the oddest or most difficult passages will have the easier questions in my experience, so remain calm, take your time while remaining on pace, and key in on what basic knowledge is this passage testing and you should be fine.