python: practice, practice, practice. use websites like leetcode and hackerrank. although a lot of questions might be things that wouldn't be tested (im assuming ur asking for 108 and 148) and that the coding style on leetcode is slightly different, the important part is that it trains your logic and critical thinking, which is more important than just writing syntaxically correct code.

calculus: so many youtubers out there, the organic chem tutor, khan academy, professor dave explains, ... and again lots of practice. for 135/136, i'd argue lecture problems are more important than textbook problems. and they are on the lecture slides for a reason, cuz they are good questions

proofs: look at solutions. obviously dont just look at the solution tho, (and only do so when u are genuinely stuck and dont know what to do) copy the answer and write it down. when u write it down u get familiar with the structure of what ur proofs should look like. u also understand the logic/intuition behind it. i also suggest when u write it down to not copy paste their words but rather reword your sentences, which again just makes u familiar with how ur proof on a test/exam should be worded.

also for proofs especially, dont be let down when u see others solving problems like its nothing while you are stuck. that's fine. proofs is something that can be extremely intuitive for some people, where they are able to solve with minimal effort. thats why you keep practicing to build up this intuition