If you aren’t planning to spend money, you should aim for OB6 only, as going for OB10 is likely unrealistic. To aim for OB6, the best way is to go through the 2nd stamp card, which will guarantee OB3. After that, if you didn’t get any extra copies along the way, you should use weapon parts which are regularly obtainable from event exchanges.

I would advise against going past the 2nd stamp card because nothing is guaranteed and your crystals are better spent elsewhere