The key thing to realize about this game is that it is not a "free forming" Action RPG (like Kingdom Hearts), as we are lead to believe when we begin playing with Lightning. Every attack is more akin to a move (i.e. Blitz) and not an actual sword strike. With this in mind, you need to learn about the strength and weaknesses of each attack (speed, sequence, length, cool down, etc.).
Once you are comfortable with your moves, you then need to learn about how to get around (assuming you aren't in RPG mode). Dashing towards and away from your enemy is crucial, as well as dodging and blocking attacks. Also, taking advantage of your terrain will allow you to move about more easily (walking up and around structures), as well as taking cover from certain attacks. Grinding is kind of pointless in this game unless you really need to get away from the enemy.
It's definitely not an easy game to jump into. You need to practice with each individual character and get a feel with how they function. Once you've practiced a character and get used to how they fight, you should be able to take on the campaign and the AI. It's definitely more of a strategical action game than a straight up brawler.