Anywhere that has lots of high-end monsters that drop many drive orbs. Personally, I just landed in Olympus, went into the first screen past Pain and Panic on the right, killed everyone in this room, collected the drive orbs, went back and boarded my gummi ship (while still in Master form) and then repeated, it didn't take very long this way to reach level 7.

The reason that you board the ship while still in drive form is that when you do this your drive gauge will automatically be maxed the next time you land so you can drive endlessly this way without ever worrying about building up your drive gauge or doing the cerberus cup trick.

Good luck!