You have to do this quest on Port Royal. Skeletons killed on other islands don't count.
Two easy places to find LV7+ Undead Pirates and Witchdoctors on PR:
- In Wicked Thicket, by the log bridge. An Undead Pirate is fighting a Navy soldier, while an Undead Witchdoctor stalks nearby.
- In Governor's Garden, in the cemetary. Lurking back amongst the crypts is a Witchdoctor/Pirate (it spawns as either).
'Course, if yer really feeling feisty, just head to Murky Hollow and attack the Brigands there.
