Question : What exactly tester(Jr.) should do if developer is referring the bug he has reported to developer as Feature or he is resisting that the reported thing is not a bug at all?
The best think is to go through the BRD or the FSD and verify the requirement of the said functionality. Once its not as per the document, revert back to the developer mentioning the specific document.
A Test Case design which ensures that every path in the program is executed at least once is a) Equivalence Partitioning b) Boundary Value Analysis c) Logic Coverage d) Random Generation