Lost Eidolons is a good game, but difficulty spikes can seriously spoil the experience. Here I will try to prepare players for what I encountered myself, avoiding spoilers if possible.

The first thing you need to do is to understand in advance, in the first ten chapters of the game, what each character can do and will be able to do in the future. But do not be afraid to experiment - most characters can be leveled up to any class. Most classes are relatively easy to open, the character just fights in battle and progresses. However, opening other classes can be much more difficult. Most often, this applies to the templar and the battle mage. This is due to the need to level up a skill that this character does not initially have access to. For example, fighters are not able to use a spell book, and can only develop a skill through training in the camp. Similarly, to open a battle mage a mage character will have to do a lot of exercises with an axe. Eden has the hardest time: he can only unlock his Overlord prestige class through sparring in elemental magic, and it requires spending leadership and are only done once per chapter, so you need to prioritize them as early as possible (although the top classes will not open until Chapter 17 anyway). Also, pay close attention to what armor the character wears and what is needed for the class. Armor is easier to train than weapons - you can just put it on for battle, but keep in mind that a character with a low skill in this type of armor initially will take a lot of damage.
A quick addition - try to fully develop your characters' preferred classes, at least one of the two. For reaching 100% in a preferred class the character is given an additional powerful skill. Some characters retain it even if they later change their class. For example the assassin skill Raid Tactics remains available to Andrea, even if she took the sharpshooter or ranger class.

For most of the game, the fights are not difficult at all, so players can completely forget about the existence of skill tab until it's too late. But you need to look there. Firstly, if the "gray-blue" skills are activated automatically and their number is unlimited, then the "red" ones take up slots in a rather small list, and as soon as the list gets full, they require manual activation in it (i.e. they need to be moved to this list, replacing some other skill). For example, a diagonal strike for a templar, or horse riding skills. Which skill to remove and which to leave depends on the character's role at the moment. A tank would benefit from additional protection, but a long-range archer would be better off taking another shooting boost. I do not recommend using two skills at all: Tactical Link: Movement and Tactical Link: Swap. They are already very situational, and considering that they end the turn of the character who uses them, they are pointless - you just miss a turn for attack/heal. It is better to use multiple variants of taunts and stuns to control strong enemies. Also, be thoughtful with weapons and armor. With the same characteristics at one tier, weapons and armor can have completely different additional bonuses. They should also be selected taking into account what exactly the character is doing at the moment and what enemies he meets. You do not need to take a gear with a strong bonus that can work once per battle, it is better to take one with a slightly weaker bonus, but which will work constantly. So, a simple "+5 accuracy when attacking" will always bring more benefit than "+500 chance to block a critical hit with dark magic if the enemy's HP is less than 50% and you yourself are jumping on one leg." The same applies to bonuses from aide characters. The same bonus can be very important for one class, and completely useless for another. If the battle promises to be difficult, take the time to properly distribute aides (keep in mind, however, that a character cannot be assigned an aide with a higher level than him).

Now to the known problematic moments:

First one is rerelatively simple - you need the maximum variety of weapons. Try to prepare a couple of battle mages or templars for this moment, and if you can't, look at what secondary weapons the archers have (or vice versa - which of the fighters shoots the best). In the initial fight with worms the purification spell will help - it removes the mark from the one they are going to eat. When final beast will appear, just surround it from all sides and hit its vulnerable points (taking into account how it will turn around). Do not forget to look at what type of resistance is hanging on the boss at the moment, it changes them every turn. This is where the variety of weapons decides. The monster is very thick, but quite killable. And do not forget to tickle it with Eden's special ability - you will get a shiny.

A very difficult mission, if you do not know the trick. The trick is to NOT do as Clara tells you and deliberately fail the initial stage of the mission. The fact is that the soldiers who come out to fight the miners take them down faster than we can advance through the castle, and at the last moment they manage to return to help their own. And this is not to mention that we lose the teammate who is performing the stealth mission (either he will be killed, or he will have to run away as fast as he can, in any case he practically does not participate in the battle). Therefore, we simply and straightforwardly knock on the main gate, listen to Clara's curses about how stupid we are, and then calmly and methodically cut out everything that stands in the way. The first part of the battle will be a little more difficult, but in the end it will be much easier.

There are many elite enemies here who come in waves. The difficulty is that it is sometimes impossible to completely destroy one wave before the arrival of another - the enemy commanders are very strong. The first turn should be waited out on your bank - let the enemy come to you. But then you need to continuously attack. The task is to have time to cross the river before the second wave arrives. If you accept the fight in the river, the current will greatly interfere, especially for the already slow sharpshooters. In the second and third waves enemy mages have area freezes, so you need to either cast silence on them or stock up on area cleansing, otherwise half the team will be stunned. Keep in mind that the enemy bishop can remove the silence, and even taunts do not help against this - healing an ally does not count as an attack. After dealing with retinue we throw all our forces against the mage-commander, because the templar is very tough and much less dangerous. The third wave is completely elite and only one regular fighter. But this regular fighter blocked 4 physical attacks out of 5 in my playthrough. However, he is well penetrated by magic. Next, we remove the bishop, assassin and mage, taunt or stun the rest, then finish off. If we played aggressively enough, our forces are in a position to help the allied forces deal with the remaining enemies.

The main obstacle for players, judging by the discussions.
You need to prepare as much as possible for this mission. Level the right characters into the right classes, give them the best gear, give them good aides and stock up on potions. You'll need everything.
You'll need more archers than usual, but you shouldn't go totally ranged, it won't help. 3 sharpshooters and 1 ranger were enough for me. It's advisable not to take pure mages and pure fighters - mages quickly run out of spells, and fighters take too much damage from counterattacks. It's better to take an Overlord, 2 battle mages and 2 templars, plus one bishop for basic healing. Battle mages are versatile enough to be useful in any situation, and at the same time are not much weaker in magic than pure casters. In addition they have a class skill with a stun. Does not work on the boss, but will be useful in the first part of the fight. Templars will provide the team with additional heals and purifications. Due to this you can take in your inventory not only heals, but also strengthening potions or haste potions - for slow archers. Templars also need to take a diagonal strike skill - this way they will avoid strong counterattacks of the boss. The most unpleasant skill of the boss - axe attack with a heal - can be neutralized by a ranged taunt of the templar - throw a taunt from afar, and then block the path to the templar with other characters. The boss will hit him with a book, not an axe, and will not heal as much. Sadly, he will still use special attacks. Sharpshooter skills that reduce physical resistance also help. I note that you only need to throw Marksman's aim, Marksman's shot is not needed - there will be almost no increase in damage, and the mark will be removed. It is also good to freeze the ground under him - although the boss himself will not freeze, he will still receive a dexterity debuff.
The boss has more than 100 HP and he is passively healed for 10 each turn, and the characters' attacks on average remove 5-6, and 7-8 in the case of a crit. However, if we did everything correctly, our characters are still 10 on 1 and most of them can attack the boss out of reach of his frightening axe. So 2-3 turns for each phase of the fight with him is usually enough.

If you have any questions, please write in the comments. Thank you for your attention.