Psychics can have different builds,
the pure build is the best build for the most damage since all psychic skills depend on the magic attack and the soulforce.
The soulforce is increasing with your lvl and the equipment you use.
for pure build you spend 9 magic and 1 str every 2 lvl.
If you want to have some more hp you can spend points into vit for the cost of a lower attack.
The Soulforce doesn't depend on your build