The best way to get the items for the subjob quest IN SELBINA which are the Magicked Skull, a Crab Apron, and a Damselfly Worm are to find friends from your guild and/or linkshell that are 20+ and mow down these mobs.
CRAB APRON: u may get this hunting with a regualr PT, which i did. They drop off snippers. They are pretty easy prey for a lvl 14-16 PT.
MAGICKED SKULL: Go with your 20+ PT and head to Mhaura. Go outside the town and head to Tahrong Canyon. Next go to the upper right corner by the crag and go to the Shakhami MAze. Ghouls are everywhere here! the drop em pretty quick. I got mine after my third kill.
DAMSELFLY WORM: Do this last if u can, the rarest of the items in my mind. Kill with ur 20+ PT for saftey but other PT's can take them too. Just kill damselfly's by the oasis or the enterance to selbina in Valkum Dunes.
hope this helps and if u cant find a 20+ PT, have fun waiting for a while :)