Spirits Within is a Weapon skill learned from Sword that can be used around...early 50s i think? I forget. It ignores DEF and does a set dmg based on TP and your HP at the current moment. So many PLDs build 300%TP and then with full HP unleash it unto the mob. Usually deals 4-600 dmg depending on TP and HP.

Unfortunetly this doesnt really make PLD a real damage dealer at all because it takes quite some time to build that much TP.