The best way to PvE with a Thief is to always try to avoid being hit. Thieves are the Damage Dealers in a PvE setting. They have a very high attack and Agility at the cost of a low defense. This makes them Fragile. Cutdown is a Thieves best friend. Always jump up and use Cutdown from above on a group of enemies to combo the group into an air combo with Aerial Frenzy and beyond. A work in progress. Add if you can be constructive.