This exploit was found while I used my quest strategy posted here.
In Old Tarantia you will get a quest that sends you into Villa Verde to rescue the owner from looters that have invaded. Complete this quest and you will get another that sends you back in to retrieve 5 valuables from the mobs still infesting the zone.
Go in and gather SOME of the valuables. They drop frequently from all mobs in the zone. Just don't gather all 5.
Go out and then do not reenter for 4 hours. When you reenter the Mobs will have respawned. Do as much grinding as you want and before you leave pick up the remainder of the 5 valuable. run outside and turn it in. get exp and be happy.
I have not verified how much time has to pass (probably 4 hours but maybe just after a relog) the npc in front of Villa Verde that gave you the quests will offer it to you again. Rinse and repeat.
It won't really result in MASSIVE xp gain. But its nice to know if you are grinding the Villas anyway.
I will update this post with more information as I get it.