There are two main ways to die: instant death effects (requiem, criticals) and HP loss. Knowing which one you're dealing with is important, as the methods of dealing with that are different. Having a lot of HP won't help you when the monster has 20 attacks and a good chance to crit you. There's also a pseudo-death where the monster kidnapps you for a few rounds and returns you with half HP or so. This is not truly a death, but it can be mitigated by the dolls with "escape resistance" like the miracle doll. Note that all the dolls are consumables and they are consumed after being used enough times. While escape resistance dolls are common enough to carry around all the time, the same cannot be said of Merlin Dolls, which should be saved for important battles only.

Also, remember that you don't actually need to fight every fight. There are many monsters where the correct choice of action is simply to run away, preferably with Flash Escape, or by using a magic wand or smoke ball to get out of there. This is particularly true the first time you meet something like a Deathseeker or Harpy Prince, given that they can appear long before it's even remotely fair for you to be fighting them.

Hearts are an important resource to manage. If you lose your last heart, you're gone for good, so characters with only one heart are in a precarious spot. The MC is lucky in that they can never lose their hearts, so they're probably the only character where it's reasonable to leave them with one heart for the bonus stats. You can restore hearts with a number of rare items, or by resting for about a week after you revive in the base.

You can actually prevent death outright with the right items or skills. This normally leaves you somewhat vulnerable, though, with only a few HP. The Merlin & Poiney dolls act as sacrifices and prevent your death. They're quite rare and they should really be saved for important endgame battles, because the shop will never restock. You should buy all 10 Merlin Dolls every game, then store them for the most frustrating battles. You will be very sorry if you waste them all on easy early or midgame battles. There are also a few rare items that revive you or restore heart points. Again, save those for endgame, they're far too rare to make regular use of them.

There are also a couple of skills worth mentioning. Fighters have Invincible Heart which gives you a chance to survive a lethal blow and clerics can cast Divine Life. It's only usable in combat and it can get dispelled, but it will let you survive death once by taking all of your MP in exchange. This works for anyone with MP, which is good because you probably want to multiclass everyone to Cleric at some point, as we will discuss next.

When you first come across them, the critical is just OP. What's worse is that many of the mobs that have it either swarm you, like Ninjas, or they get to hit you a whole bunch of times like the aptly-named Deathseeker. However, there's a secret to dealing with crits: they count as a status effect, just like poison or silence. The same goes for the few mobs that cast Requiem, it's an instant death status effect. What good does that do?

It means that Spirit Wall stops instant death from working, which is likely to be your only real defense against it for quite some time. Later on, your knights will get Secret Defense which is good, and your front liners may already have Invincible which is nice, but not perfect. But Clerics get the best skill of all: Holy Soul. That blocks all status effects, even instant death.

Okay, but what about the monsters that just hit you 8 times for 200 damage each? The preventatives are too rare to deal with this and being immune to status effects doesn't really help with damage.

Here, probably the most reasonable thing to do is simply to debuff the mob to hell with the ranger skill Confine, while buffing your own avoid stat with Mind's Eye scrolls (preferably spammed with trick), or master casting Multi-Avoid once you have a master mage/cleric.

Class changing into Knight is also worthwhile for a ton of bonus HP and the Trained Defense skill. Ninjas have a nice skill called Evade Training as well. High AGI will help you dodge, high VIT will give you defense, though only permanent VIT gives you more HP.

Finally, Black Wall is nearly mandatory on some of the boss fights. It may be worthwhile simply to have the front lines defend, while the back line debuffs the monster until it can no longer hit anyone. The cleric skill Holy Shield also deserves a mention as it's great vs. the monsters that hit you only a few times for huge damage, given that it always negates at least one hit.

Ultimately, this may require a bit of grinding if the monster is simply beyond you, but once everyone has a lot of HP and you can debuff quickly via Trick 2, Master Cast or Confine, the things that were too hard to handle suddenly become trivial.

End game, though, you have to boost avoid through the roof (Miracle, Trick 2/Master Cast of Multi-Avoid, etc.) and one of the two Veil divinities will be almost required. If your front three people are all ninjas that are hidden, the only real threats will be requiem (which you should counter with Holy Soul) and dispels, for which you need to keep Force Guard up.