As with every other Final Fantasy out there, save for VIII, there's no such thing little bit of grinding can't fix. X doesn't really have much depth when it comes to battles, or so I feel, most bosses don't even require special strategy to beat, grind a little bit and stick to what Andromeda said and you'll do fine. You could also try to obtain some of the Celestial weapons, those would reduce difficulty quite a bit, though be warned, you need a lot of patience and nerves to obtain most of them.