I'm an A2 student currently doing Maths,English Lit, Chemistry and History and i personally found at the beginning of As a big jump even though i had all A and A* but with good revision techniques i'm predicted As in all my subject. One advice pick a subject that you enjoy and are passionate about because i chose my subjects because i genuinely found them interesting hence why i performed better. With Chemistry and Maths it is a big jump from gcse and working hard from day 1 will result in the grade you want at the end of Alevels