First Vanguard Rider Sabitus:
The main thing about Soulshots is to have them always at your disposal.
You don't have any in your inventory, right? Then I'll give you some.
Soulshots are equipped automatically when you start using a weapon.
Also, you can come across Spiritshots - these are like Soulshots but they increase your magic powers instead of fighting abilities.
Well, why don't you test these Soulshots yourself? Use them to attack Training Dummies.