I know I know: studying the theory, experience and years of practise are essential. I know it, you know it.
However I am pretty sure there must be some particularly good resources (books, pdf books, pdf files, websites or what have you) or strategies (mathematica/matlab maybe? Stackexchange?) to improve "computationally".
By computationally I mean being quicker to evaluate things like integrals, ODEs, PDEs, integrals that you can solve with complex analysis etc. So, all those things that we all can do after a bachelor, but that allow people to sit on very different points of a scale. Some people are way way way better than others because they have practised more or had learned tricks. So my question is
What books, pdf files, websites, strategies, resources or tools (the most important would be books for me!!) can someone use to MASTER and PRACTISE (a lot, as in thousands per type if not more) the above computations (and related)?
Some problem books are very useful for that. For instance, since you mentioned PDEs, I would suggest Claude Zuily's "Problems in distributions and partial differential equations" https://www.elsevier.com/books/problems-in-distributions-and-partial-differential-equations/zuily/978-0-444-70248-7.