I think you should learn it properly even though it will take more time but you do need to know it all ultimately. I would suggest going through textbooks/revision guides and something that helped clear my concepts were youtube videos. Freesciencelessons, Science with Hazel and myGCSEscience are AMAZING channels which go through pretty much the whole specification. So, I'd definitely recommend them but other than that practice as many past paper questions as you can so you get used to exam style questions and know how to approach them. Good luck with your mocks Xx