If you find yourself dying often don't be discouraged. It's going to happen.
Unless of course you listen to the following 10 steps. Then maybe you won't die... as often anyway.
1. Use 'L2' and 'R2' to switch between commands
If you're anything like myself, and apparently the majority of first-time Ni No Kuni players, you'll find yourself switching commands with the D-pad. This is a massive pain in the ass, until you find out that you can switch through them with the shoulder buttons.
Using L2/R2 to switch is not only more convenient, it's much quicker. Which is important when you have to Defend in an instant, something we will get to very soon.
2. Learn the Elemental Signs
These are important because they are stronger (and weaker) against other Elemental signs. The order is as follows:
Sun beats Moon, Moon beats Star, Star beats Sun.
If you pop into battle with a Sun Elemental type and are facing a strong Moon Elemental you'll have the upper hand. Use this to your advantage. At the same time, pay attention to who you are fighting against for defensive reasons. Don't throw your Moon Elemental out there against a horde of Sun Elementals. You'll die before you even realize what hit you.
This is by far one of the most important lessons you are going to learn if you want to survive in Ni No Kuni.
Update: Thanks to 'Pazno' for pointing out that I forgot to mention that 'Double Planet' beats every Elemental with the exception of 'Single Planet'. Thanks!
3. Save Often
Which is why it's important to save often. This way you won't have to spend your money and will only have to load a saved game from 10 minutes ago instead of 10 hours.
4. Capture Everything
Ni No Kuni is like Pokemon on crack. There are 400 monsters to catch, each of them offering you a different set of attacks, strengths, and weaknesses. Once you gain the ability from Esther, it's time to catch 'em all.
The more creatures you have, the more options you have. The more options you have, the more #winning that will happen. There are also some side quests that require you to capture specific creatures.
Bottom line: capture them all dammit. Don't be that guy.
5. Don't Get Attached
Your first Familiars aren't your strongest ones. Neither are the "cutest" ones. Instead, look for Familiars that have a high Attack Speed and Movement Speed. Those are the ones that you want.
Sure that 'Hooray' you named 'Hip-Hip' is super cool, but it's necessarily the one you want in your corner.
6. Use Magic
Don't be afraid to switch to Oliver and use your magic. It's saved my ass more than once, especially against bosses. Use the quick switch to Heal and cast offensive spells.
The trick here is to bob and weave constantly. Never stay still. Get distance, cast a spell, get more distance. Repeat this until you win.
Keep in mind that not every Familiar has the 'Defend' command, but Oliver always does. If your Familiar isn't equipped with it, be ready to be quick on the shoulder buttons to change to Oliver and call a defensive stance.
8. Don't Always Early Metamorphose
Doing this will make them the strongest they can possibly be. Transforming them also takes them back to level 1, so be careful. You don't want to do this in the middle of a dungeon or right before a tough boss fight.
9. Finish All Side Quests
Side Quests aren't just there for extra gold and a few new items. They are to unlock Merit Stamps which unlock some serious perks. Higher Experience per battle, faster movement on the map, more health drops during battle, and plenty more.
Take a few hours out of your 100-hour journey and finish your Side Quests. Especially the Bounty Hunts. You'll be thankful later.
10. Situational Awareness
The combat system in Ni No Kuni is a combination between real-time and turn-based. You can move around in battle and dodge enemy attacks, but you also have to wait your turn to attack.
With that, it's incredibly important to understand the situation you're in. Know your distance, your enemy, how much health you have, and which Familiar you need to use to gain the advantage.
I have beaten more than one boss with Oliver alone by simply keeping my distance, defending when necessary, and unleashing a barrage of spells. Almost as important as understand your Familiar's Element is understanding your surroundings.