In India,Written test for Combined Defense Service (after graduation) is very easy. A reference of previous question papers is enough.The screening test,psychological tests are not big problem.
But real test comes in the physical fitness part. There are some training institutes in India who gives specific training for these tests. I have seen my stout build friend from Nagaland failed to pass even after completing all the tasks in time,because the one who got selected repeated some tasks twice/thrice.And if you are applying for flying branch, I don't know any training center for PABT.
PS: If I were you I would have joined the training centers because some days never come back.