Quantum field theory is one of the most complex things I've ever done. There are enormous amount of derivations with hidden and nifty tricks to get it done. People who devoted their life to work in this area must be had a great computational and marksman skills to take care of all the indices, especially if working with traces of gamma matrices in scattering problems. Haven't done any renormalization and path integrals, but I guess it's enough for now. At least I know how to proceed with QCD.
Pro tip: combine this book with Peskin's and Greiner's. Deadly trio.