Couple really kind and beautiful ways to get early XP boost...

Number 1.)
Kill the 2 magisters that you meet at the beach. They are level 4 though so you better be prepared. Killing them does not seem to effect anything and with good luck you may get some nice gear off from them.
Number 2.)
If murdering innocents comes naturally for you... Slaughter the elf fool who is in the hidden alcove for few more extra XP and tasty elf flesh bloodcake.
Number 3.)
Give in to your dark ways and without mercy decimate the two magisters guarding the entrance to Fort Joy and revel in extra XP and their guts.
But be prepared... These dark and secret ways to get you forward may also get you in trouble.... But, if you are evil you always have the option to not care of trouble and destroy everything.