If you have trouble with battling, this can help you.

Tip #1 Using Psynergy Wisely

If you get hurt during battle, remember that you always have items that can cure you, like herbs, nuts, vials, potions, etc...
If you use items to cure allies, then you can save your Psynergy for other uses.
Only use Psynergy if you are seriously hurt. (Like Potent Cure and Ply Well)

Tip #2 Djinn

Unleashing Djinn can inflict more damage than Psynergy.
So, try to find all of them! You won't regret it!
Djinn can attack, defend, cure, revive, and do lots more!!!
The more Djinn you have, the more powerful you are.

Tip #3 Special Weapons

You can find special weapons in chests.
If you equip them, your character will now be able to do a special attack at a random moment if you select "attack"
For example, the Bandit's Sword (You obtain by defeating the gang of thieves) can unleash a special attack called "rapid slash", I think.
So, you should try to open all of the chests in each area.

Tip #4 Item Storage

You should always have many herbs, elixirs, and antidotes in stock because you will never know when your healer is down or if you get poisoned, fall asleep, or get delusional.
You should also keep rare items like Water of Life or Psy Crystals because those are important.
Spread your items around the party. You can't just have your preferred character have everything!

Tip #5 Rest Well and Save Game Before Fighting a Boss

Before fighting someone tough, you should sleep at the nearest inn and save your game right before the boss fight.

Tip #6 Preparation for the Final Boss

IMPORTANT!: Before fighting the final boss (Which is really tough, by the way...)  Complete this checklist:

1. Make sure you have full HP and PP

2. Set all the party's Djinn so they become usable

3. Make sure you're completely satisfied with your allies' levels (If they're pretty low, flee the area and go slain some more monsters)

4. Make sure you have enough Djinn

5. Equip your team mates with the best weapons and armor

6. Make sure everyone has at least two herbs or nuts or vials, two antidotes, two elixirs, two Water of Life, and one Psy Crystal.

7. Save the Game

8. Bring it On!!! 

FINAL TIP:

If you are fighting a tough foe, than make sure you use a Djinn that can boost your resistance against lethal attacks.
Use some Djinn to attack and make Mia your healer.
Once you start off, make sure you've got four Djinn (or more) on standby so you can summon something powerful.
Make sure you also found other summons, too.
Keep on using Djinn and summoning and using Mia to heal.(Because she's bad at attacking)
You should also use Djinn to boost your attack, agility, resistance, and to cure the allies' statuses.
Use herbs to cure minor damage and elixirs to cure sleep.
You can also use "cure poison" and "restore" to perform the same actions that antidotes and elixirs do.
Once you beat the tough foe, congratulations!
I hope this helped.  Bye.