This is how I did it:
1. Enter Bayjin Shallows, kill big momma get excaliber.
2. Save before fighting Bucanner ghost.
3. Fight with ghost, took out 3-4 of them and blinded the rest with Omniguns and Ebon staff.
4. Let them retreat and save the game.
5. Retreat to the nearest corner and wait for them to come back. Ambush and kill them all.
If you are unable to get a *Light* *Sword* from step #3 or step #5, reload savegame from step #4 and replay the combat in step #5. You will need plenty of retries to get the light sword but the battle will be significantly shorter than the one in step #2.
Hope that helps. Oh... trust me, the sword is *worth* the trouble.