After you get a realy high str. Lv. You need to get more team members switch jobs and then got train it will be easy if your str lv is up.

If your a fighter or a thief you can cast any spell if you make both of your subjobs mage and preist. But to regain more HP put priest first.

If you take the job fighter or thif you can make both of your subjobs preist then learn aid, parelell aid or cure. After the next battle go to your pause screen, go to magic select the spell that regains HP and then you will be all healed.

If you change your job every time you get a THOUGHT POINT you can get higher stats.