[Caspian]

In a solo situation, sub whatever you want. For parties, you need to either sub monk or warrior. Your job as a THF is to do as much damage as possible. Mage subjobs do not help with your damage output. Sub warrior or monk until ATLEAST lvl 24. At that point you can make WHM your main and use it to unlock the Ninja job. Then you can either buy or quest Utsusemi: Ichi. This is a spell that gives you 3 shadows everytime you cast it. You'll need a shihei powder everytime as well. What would normally be 3 hits to you will instead just take shadows and cause no damage to yourself.
Do a search on somepage for it.