1. There really isn't a way. KOing the countless Gastly in the Old Chateau is much more efficient.
2. IMO, BW are easiest.
3. Who knows?
4. Depends on your team and what attacks specifically you want that Latias to wall.
5. See #3
Thanks for the info! Also, are there any sites competitive battlers use besides Smogon and Nugget bridge?