The way I leveled Master form (in the later stages of the game) was to transport to "The Lock" in The Underworld, go through the door and Change to Master to quickly beat the 3 guys in here, then head through the next door and drop down to beat the 3 guys down here and collect their orbs. Thunder Spells are the quickest way to nuke them, Thunder Boost obviously helps.
Obviously, the trick to doing this is killing the Heartless and collecting the orbs fast enough to get back to the save point fast enough to transport back out to the world map before the form expires.

I expect Tavrobel will be around to set us all straight sometime soon.