My strategy for the international law was only one book "International law by agarwal" i read each and every law with due focus and answered the question as demanded

for US history youtube channel "crash course" and book by majumdar. For us history i skipped the not so important presidents and chose to study imp ones.

Psychology again youtube "crash course" and fpsc recommended book. just focused on the contents that is it (diagrams can help you get good marks, i drew the diagram of neuron and the brain)