The most useful ally is Kiara (the trap). He is the only one who can use necromantic spells. Those spells doesn't use too much MP and has a lot of useful spells against bosses.
One spell damage depends on the target's max hp. It hits around 10k to the final boss.
Another spell gives the target a debuff in which they receive 50% extra damage from a physical attack.
Another spell makes all the enemy physical attack hit like wet noodles. If you combine this spell with the debuff dragon that you get at midgame, your team is nearly invincible.
The second most useful ally is Eleanora. She sucks a lot in early game, but once you get the debuff dragon and the non elemental magic attack dragon, she will be a good mob cleaner and boss killer. She needs a lot of MP to spam those moves every turn. My endgame Eleanora was spamming her ailment dragon spell every turn against the final boss., but she had 100 max MP.
Before she gets her good dragon spells, give her a bow and make her fight at mid line. She has decent atk stat and the bow skills hits quite hard.
For Ayana, learning some unarmed moves will make the lategame content easier. Non weapon skills can be used with any weapon, but you can only learn them if they attack without weapons. The skills to learn are the one that spins the enemy tand throws them and the one that fully charges the tension bar in exchange of some damage.
You combine the unarmed moves while equipping a staff and you will hit a lot every turn without caring about MP. The strongest staff also has a very high atk stat (a few points less than the ultimate one handed sword, that's how broken that staff is).
As you kill more monsters, higher tier spells will be unlocked, but their damage is still trash in comparison to physical skills.
Magic skill main advantage is that you can use them while having the staff equipped so you can dish mid tier damage all the time. Skills in this game consumes way too much mp and there isn't a reliable way to replenish it (suicide skill from Elise and a dwarven drug that gives you a confusion debuff).
Since the mp regen for staff users depends on their max mp, you will want to have high max mp obviously. With around 70 you are more or less fine, but with 100 you have an unlimited amount of MP. You can bring Elise whose tension skill will kill her in exchange of fully restoring the party's MP. If you teach Elise the unarmed skill that fully charges tension then you have an unlimited MP battery. The final fire spell is a revival so is a decent combo, but it wastes too much turns so is better to have high MP.
From mid game, the dragon summoning or the high tier magic spells will make most encounters end in 1 turn, but if you do that you won't be able to farm for skills. It's such a stupid game design.
On the other hand, the endgame unique staff atk stat is so high that its better to learn some non-weapon skill and use them instead of magic (you can use non-weapon skills with any weapon). I spammed the move that spins and throws the opponent against the final boss with Ayana all the time thanks to that endgame staff that has insane atk stat. The formation was:
Ayana alone in the front line; Eleanora, Kiara and Elise on the backrow.
The lewd elf dragon skills sucks until you can kill the 3 special dragons. One of them hits a lot, another one gives a shit ton of ailment making every boss a joke (it even works against the final boss) and the third special dragon is useless. You can get other weaker dragon spells by killing dragons, but the only random dragon I found was the ice one from a guild quest.
To unlock the special dragons, you will need to follow Kai's storyline:
Go to the cottage at the lake in the mountain road, talk to the old man, go to Dumwart (or however that town is spelt) and take the boat at the west side that leads you to a fire dungeon for 2k gold (there is 2k gold inside that dungeon), meet Kai in the dungeon, return to the cottage at the lake, return to Dumwart inn and recruit Kai, go to the cottage at the lake and go to Svern town. At the end of the cutscenes you will unlock the 3 dragons.
1st dragon in the dungeon you met Kai.
2nd dragon in another fire map (the one next to the dwarf city and leds to the orc village). This one is dangerous, has an AoE spell that hits more than 1k and I honestly don't know how to survive that, I just killed it quickly using all the powerful skills and spells I had at my arsenal.
3rd dragon in the shrine south of the Valkyrie village.
The game is not hard (I was playing at 200%) difficulty, but is not exciting either.
A final tip. Buy water from a merchant and drink it to get an empty bottle. Use that empty bottle to make Ayana pee inside it and you will get a monster repellent item. It makes the game easier, especially once you have already unlocked all the final tier spells since you won't need to fight anymore.
