That's the most usual one... I use a bit different method, though.
1. Go to Hollow Bastion > Choose the "Merlin's House" -save point
2. Stop in front of the door and transform into Master Form (I do it always there, because in a "save point" -room you'll never transform into Anti Form)
3. Go outside. Enemies start to appear immediately (Bookmasters). Kill them and take the Drive Orbs
4. Head towards the route straight opposite Merlin's house's door. A Morning Star appears > Kill it, take the big orbs.
5. If enough Drive left, go to the place where you met Yuffie the first time you visited Hollow Bastion, just around the corner
6. Two Bookmasters + one Morning Star appear > Kill them, take the orbs, and go back to Merlin's House. Stay in Master Form all the time.
7. Go to the save point > choose World Map -option
8. When in World Map, go back to Merlin's House. Your Drive Gauge is full once again.
9. Repeat, if necessary.
When you get more Form duration, you can continue the route from the place I mentioned in part 5 by jumping up the machine -thingies, and killing the Bookmasters, which appear when you reach the top. After that, go pass the stairs that go down (towards Bailey), and a Morning Star attacks you once again. Kill it, take the orbs, and jump down. Near the stairs one more Bookmaster appears. After slaying it, go back to Merlin's House.
That was pretty badly explained.... XD