You need to use high level berries.
around victory road. never use a ford Or in sky pillar.
Go to the contest hall in lilycove and talk to the lady on the left. She would give you a poke block case.
You can't reduce a pokémon's level.
insert the sapphire cartlige in your ds then the diamond and turn it on. Go into Pokemon diamond and an option should be migrate from sapphire. If this does not happen, there is a possibility that...