You have to find a shiny stone (which are found on iron island b3f or on route 228)

Unfortunatley you cannot find misdreavus in pokemon platinum so if you or a friend has pokemon pearl then you can catch it there then trade it. Oh you it is in eterno forest in pearl

Well in pokemon platinum scyther doesnt evolve by level. You will have to get the metal coat from Byron on iron island. After you do this you can then make scyther hold the metal coat and then finally trade it with a friend so it can evolve into scizor