How to level/play efficiently?

[GoingMyWay?]

I'm not exactly new to RPGs, but this is my first Etrian Oddyssey-like, and from what I've gathered it's easy to screw up both general combat strategy and character levelling. With the fact that I'm more used to "static party" RPGs (and not the amount of character switching mid combat I've managed to gather occurs in optimal play) and the rather expansive ways to strenghten characters, I just wanted to know what are some tips and tricks both for combat, and for efficiently strenghtening characters?

[Corence]

Level up bonuses, skill points, and subclasses can all be reset freely so feel free to play around with that. In terms of efficiency, I think you get the most value out of putting level up points towards stats the character has naturally high growth in versus trying to shore up weaknesses. Like a Tank Komachi should invest in HP to supplement her great HP growth rather than DEF and MND.

This wiki has some advice on different character builds which might give you an idea about how people think about building characters:
https://en.touhouwiki.net/wiki/Labyrinth_of_Touhou_2/Characters/Character-Building_Suggestions

There are also additional details on character pages about how their skills change as they level up, etc: https://en.touhouwiki.net/wiki/Labyrinth_of_Touhou_2/Characters/Characters_1

That wiki is just a great resource in general to check out.

Otherwise for combat, a big thing to realize is switching characters puts the switcher and the newly brought in character at 7500 which is faster than attacking and most spell cards. This means swapping a character after they attack is more time efficient than waiting for them to get another turn and having them swap themselves.

I've felt like the game does a decent job with the bosses in early floors testing your ability to adapt in a few ways. One nice thing about LoT is that it's forgiving -- most bosses are quick to access from a teleport location, you can immediately retry a boss if you die, and there's no penalty for wiping aside from returning to Gensokyo. So hopefully that feels a little less like "easy to screw up" and more that there's a lot to learn but the game gives you freedom to try things out and learn what works well.

[Incognito]

The important thing to know regarding stats is that consumable gems and the "boost" skills improve stat growth by a flat value so they're worth proportionately more when used on weaker stats. Level-up points, library levels, and equipment bonuses all contribute to the same bonus multiplier. Investing points/money in improving Komachi's DEF for example is worthless because it'll always be multiplying a very small number.

Because the library costs increase geometrically, it's going to be much more efficient to have some investment in everything you might use instead of fixating on the best stat. Conversely it may be better to dump 10 gems into each stat you want to improve instead of spreading them out, because a bigger improvement to the base value means you get more mileage out of investment.

Also, library investment in affinities is multiplicative of the base value too, so trying to use investment to improve a naturally bad affinity will be expensive. Affinity equipment is always a flat bonus, so use those instead until you're swimming in money.

[Brightside]

My first time through the game so these are some of the things I learned so far.

When regularly dungeon spelunking just look to end fights ASAP even if being MP inefficient. AP is the main thing holding you back when doing regular spelunking so having a regular rotation of chars dropping AOE's to end fights quickly is a must so you aren't running in and out all the time. The AP reduction only occurs on your front line fighters so if one of your chars gets heavily damaged just switch them to the back instead of trying to heal them. They won't suffer from the large AP reduction from having to heal after the fight if you do this.

For bosses pay attention to what elements they are using and if they are doing Phys/Magical damage. A lot of bosses lean heavily into specific elements and the game usually provides you with equips that raise those specific elemental defenses. Sacrificing some of your damage output to ensure a stray hit doesn't 1 shot your slot 3/4 is critical.

Buffs are huge and Reimu's defensive spell in particular is good at ensuring some weaker chars survivability. Managing buffs is important and if you switch a char to the back row their buffs don't decrease with time until they come out again. Doing this you can buff a char and hide them in the back until you need the sudden increased damage output.

Status effects are both extremely debilitating to your party and effective vs enemies/bosses.

Be flexible with your party composition. Every char isn't effective vs every boss so you'll need to get used to dragging around wildly different chars depending on what your fighting.

[...]

Wiki page mentions resetting library points. Is there any particular reason to do so? There's no in-game description of them being somehow limited.

[Balsion]

Generally speaking, you want to focus leveling bonuses into 1 or 2 stats. Attackers should just put all points on their main attacking stat, Buffers or other support chars will probably want a mix of SPD and HP, Tanks can go for a mix of HP and DEF/MND, or if you know an enemy only uses attacks that target one of the defenses just put everything into that (exception would be chars like Komachi with good HP but bad defenses, in which case just go HP all the way).

For library, you're supposed to invest money into all the stats, except into whatever offensive stat the char doesn't use (there are some hybrid characters that use both offensive stats, in which case no stat should be neglected), for example, Marisa doesn't really use the ATK stat, so she only needs library levels in HP, MAG, DEF, MND and SPD. You're technically recommended to have 1.2x the char's level in library levels for the regular stats (i.e. if you're level 50, your library levels should be level 60), but in practice you won't always have enough money for that, so something like 1x your level for the most important stats and 0.5-0.8x for the other stats is generally fine. Affinities are separate since they're much more expensive, so you should just raise them whenever their cost is about equal to raising one of the regular stats up.

For equipment, keep in mind that equipment bonuses all add to the same multiplier, so you get diminishing returns if you only focus on one stat. So for your attackers sometimes it's worth it to use 2 offensive equips + 1 defensive one vs a full offensive loadout.

Originally posted by ...:

Wiki page mentions resetting library points. Is there any particular reason to do so? There's no in-game description of them being somehow limited.

You can reset library points using a Tome of Reincarnation. It's mostly to get the money you invested in that character back so that you can transfer it over to a different char; like maybe you've been putting library money into Keine but now you recruited Ran and you like her better, so you get everything you invested into Keine back, and can now give it to Ran.

[Remi]

For level up bonuses, focusing on strengths is better than trying to improve weaknesses. Trying to increase Patchy's HP so she can maybe survive a basic attack takes a lot of points which if invested into MAG or even MND instead grants her a much higher ROI.
Putting points into SPD will likely not make a large enough difference outside of a few very high speed characters which can use the stat optimally.

For healers, their heal strength scales with MAG (I don't think there's a single dedicated heal spell with an ATK% scaling), so depending on how high the MAG% in a particular healing spell is, you might need to adjust the MAG investment to heal a sufficient amount.

Tanks with high DEF/MND and investments in those stats and some buffs can easily nullify incoming damage at the earlier stages of the game, but HP is usually the safest option to put points into, increasingly so as you progress in the game. Even early you might run into enemies that can bypass defenses so HP and affinities are the only stats that effectively reduce that damage.

As for swapping characters in fights, the frequency of switching really depends on your party composition. If you run glass cannons then you'd want to have 2 or 3 stay-in units that buff and switch the glass cannon in and out before the boss gets a turn.
Good stay-in characters would be tanky buffers/healers and status/debuff inflicting units, as well as characters with powerful passive skills e.g. '30% reduced damage taken from x element for the entire frontline'.
But bulky attackers can pretty much sit in the 3rd or 4th slot with as many buffs as possible and fire off low-delay spells.

The wiki linked in a comment above is a really good resource if you want more precise info on things like spell formulas and other numbers and percentages.