Hi,
If you have 12 hours of downtime than why not perform offline reorg.
For Online reorg cannot be done in batches -- not required, as during online reorg your table will be used by the system/users as normal (No downtime in other sense)
The only problem during online reorg is it will run for longer time and may have performance problem as table will be live and usable.
You can try to schedule reorg using DB13 or directly at OS level.
Regards,
Prithviraj