Use battle help in the menu to learn what all the status effects do.
On a fight that you're struggling with, turn off auto-battle, reduce the speed from 32x & use the "check" command to see what the enemy is doing to you, is immune to, high armor, high dodge etc.
If you have auto-party build set, it will automatically select a preset build for each character in your party. Your talent points will be automatically spent as you gain them.
Pyrrhic victories (where you & they both die at the same time) are still victories.
Rings apply not just to basic attacks, but also multi-hit skills and AoE attacks.
Some characters' auto-set AI seems to be bad on purpose to teach you how to edit it.
Cleanse always special, Huntress always skill, and sometimes using Tankston's special will make a really big difference.
Set the healing threshold really high rather than 50%. This way in multi-wave fights, you won't start half dead.

How does Thor work?
His skill turns himself into a character.
His special turns everyone into a character.
Any time in this guide you use Thor, set skill to always & special to never.
The copy (bottom right) section of the auto battle menu sets who he will copy.

Spirit is best spent "flat/shallow" rather than "spiky/steep" while ignoring whatever status effects your party isn't using.
Recognizing when to reset your "generic build" sprit tree to focus on spiking a couple things because so many of the other things do nothing is useful.
Other than the mage town, you can full clear every town upon arrival (except maybe the arena).
Use the other guides on steam for these.
The rewards are awesome!
Spirit fishing is infinite. You do benefit from fishing more spirit, but there's diminishing returns.
I stopped at 20 cumulative when I got the max tier of the associated item upgrade & I wouldn't recommend grinding much more than 200 if it's still fun for you that long.

Leveling up by playing minigames is always an option & it will always help.
However, if a fight is green, but you're still struggling, that enemy is probably a "boss".
The auto party build would take days of grinding to dummy thicc their way through these fights without strategies.
Read on to learn these strategies!

I'm assuming that auto-party build was able to clear your map to look something like this:

Blair & Morgana:
Auto party build should be able to beat them on standard mode, but in modes where they can have healing items equipped, they can be a problem.
Use Buffy + Thor copying Buffy, and make sure Buffy is either set to always special to hit the whole party or target: highest attack & that the specific party member you wanna buff has the highest attack.

Right, slightly below center is the armored slime.
1) Replace Ticktock w Buffy from the autobuild
2) Equip bleed ring to Huntress (play w Chop a bit to unlock it & max it out if you haven't)
3) Make sure the auto battle menu is set to always skill Huntress & always special Cleanse
4) Reset spirit tree to go all-in on bleed & basic attack damage
5) After winning the fight, reset your spirit tree again to be evenly spread across everything you care about

Bottom right is the healing slime
Equip bonus stack staff to Nyx & do steps 1-3 of armored slime fight.

Necromancers:
Either kill them all at once using big number Froggy special by doing Buffy + Thor, or reset your spirit tree to invest more in lightning stacks so that the adds they summon die instantly.

Ghosts:
Cleanse special will throw chill back on the ghosts, which stops them from dodging.

Vengeance enemies (ones who deal massive damage on death):
If it's only one wave & you die at the same time they do, you still win!
See Necromancers above for the kill them all at once plan.

Dragons:
Equip elemental rings to Hearth or tanks as the fights warrant.
Specifically lightning on tanks & single target statuses on Hearth.
Set Huntress to always attack most health. Froggy special should kill adds.

Jake:
He's getting a full loadout per difficulty treatment in a later section

Armored Slime w Bleed Ring:
Can't debuff the enemy? Buff yourself instead!
Replace Ticktock and Nyx with Buffy & Thor.
Equip extra stacks staff to Buffy & + damage to Froggy & Huntress.

Giant Ghost w Cold Ring & self-healing:
Respec spirit tree to focus on elemental statuses & *unequip* elemental rings.
Cleanse always special should outrace it.

Massive HP enemies w Vengeance:
Pyrrhic victories are victories!
Big number Froggy got you to kill all enemies at once.
What's big number Froggy?
Replacing Huntress and Nyx with Buffy and Thor set to copy Buffy
If there's another wave, Revival Draught on Hearth

I had 200 spirit in all game modes except evil, which I did the 50 floor arena for another 200 in like 10 minutes using the same build I used for Jake, while skipping waves along the way & returning w more spirit from later waves.
I had 50 talent points in standard, 90 in tougher enemies & double trouble, 140 in evil.

UPDATE:
People keep asking how to allocate talent points.
Generic auto-assigned talents are good enough.
For each character, select a pre-set option.
The majority of these selections don't really matter which one you pick, just *that* you picked.
On high difficulties, Hearth set to AoE-rez, not only heal.
If their options are skill/special/both, match those w what auto-battle options are set to below.

Tougher enemies & all easier game modes: (50 talents for standard/idle, 90 for tougher enemies)
Thor is set to copy Hearth.
Since I used Tougher Enemies for this build, I had an item equipped that you do not meet the unlock conditions for on your first file.
Substitute other items or possibly grind just a little bit more on standard.

Double Trouble: (I used 90 talents, but it was easy. 70 or 80 should be doable)
Spirit tree - flat first 4 & bleed, rest dumped in poison
Note that you may have rolled multiple waves of Vengeance enemies.
Vengeance is dodgeable, so manually tweak Hearth's talent points to maximize dodge.

Evil mode: (140 talents)
Spirit tree - flat first 4 columns, rest dumped to lightning
Note that in evil mode, you may have rolled multiple waves of Vengeance enemies instead of one mid & one final like my seed did.
Vengeance is dodgeable, so manually tweak Hearth's talent points to maximize dodge.
I have also heard good things about using Shadow Scythe on Hearth, turning off autobattle on the particular waves, killing the vengeance enemy with some adds surviving & rez'ing + healing before completing that wave.