Greetings to all of you who visited this guide. Welcome!
I’m doing my best to make it comfortable here.

There was barely any info about all the achievements, so I had to spend an unreasonably long time to get them. Here I share my findings so that you can spend 10 times less.

Let’s get started.

Story achievement

Complete the game.
Difficulty: Easy, unmissable.

It unlocks when the end credits begin.

All the other 12 achievements can be easily missed if you are not aware of them.

Play with the main menu buttons for 3 minutes.
Difficulty: Easy.

Anytime in the main menu.

Beat every enemy in the game.
Difficulty: Medium.

First, here is a short info note the game itself doesn't explain.
It will be useful for this one and for other achievements too.

The Checkpoint (a.k.a. Breeding Crystal) spawns Lumini from energy you have collected, the color of spawned Lumini is based on your previous actions in the game:

• Stunning or attacking enemies gives you red Lumini
  
• Evading enemies gives you blue Lumini
  
• Collecting the yellow orbs gives you yellow Lumini

Now about the achievement, the only part to pay special attention to is the beginning.
Purple Lumini can't beat an enemy. So you need at least one red to spawn at the Checkpoint #2 (since #1 in the starting area can only spawn purple). Doable with #3, but the sooner the better.
To get it, use the purple stun skill as much as you can.

Once you got one red Lumini, go back and clean out all the enemies you left behind within a previous checkpoint(-s). Don't progress too far forward and don't reload from checkpoints before this part is done. Otherwise you'll find closed doors blocking your way back to some enemies at the Checkpoint #1.

The rest is easy, keep beating all enemies you meet. Good news, they are not hiding and you can hear them from far away.

It's easier to get on Normal - much less enemies and it takes twice less hits to beat each one of them.

Evolve your red Lumini, it helps too.
Upgraded once, red skill hits and pushes an enemy away from you.
Upgraded twice, it deals 3 hits instead of 1 before pushing away - that is enough to beat any enemy on Normal.

You will get your achievement by the Checkpoint #16.

It's not necessary to beat them all at one go. You can save and reload checkpoints and all your playthroughs count towards this achievement (I got mine in the middle of the Checkpoint #14, when I still had a bunch of Parasini jumping around at that playthrough).

Enemies

Bring a dead enemy to the Sanctuary.
Difficulty: Easy.

You need at least one red Lumini for this achievement.
The Sanctuary is after the Checkpoint #3, not a hidden area so you'll see the place.

Lure a Muncher from the previous cave as close as you can to the way down. Alive it will not go there, but a dead enemy falls all the way down nicely.

You can also push it with your blue Lumini dash skill if it landed not too far.

Follow it down to the Sanctuary, here you'll get your achievement.

Play the Lumini theme using the note blocks at the Sanctuary
Difficulty: Easy.

You can get it at the Sanctuary, right after the previous achievement.
There are 7 note buttons on the left wall, push them with your Lumini in this order:

Beat 5 enemies with a bomb.
Difficulty: Easy.

The goal is to beat any 5 enemies with explosions. Most likely, you will get it just playing first time normally.

Preferably small enemies like Parasini or Fricatcher plants, on Normal difficulty - it takes less hits to beat. But up to you.

Fungi-Bombs are very common starting from the Checkpoint #9 and all the way ahead, just look at the ceiling or the trees above you.
Push it with your Lumini, or purple stun skill, or red hit skill - everything works.

Don’t forget to save your swarm, explosions are dangerous for Lumini too.

There are 4 achievements you can get for a game completion (other than unmissable one) - each of them pops up only after the end, when the credits begin.

Conditions are:

• Complete the game in Hard mode
  
• Don't beat a single enemy
  
• Beat the game with a purple Lumini
  
• Don't lose a single Lumini

It's up to you if you want to combine some together or go for each one of them separately.
More about them below.

Complete the game in Hard mode
Difficulty: Easy.

Hard mode is unlocked after beating the game once. You have the option to select it when you restart your game.

There will be more enemies and they will be faster and twice tougher: for example, a Muncher on Normal - 3 hits, on Hard - 6 hits. And that’s all, no new hazards or map changes.

Basically you just need to run though the game second time, no other restrictions.
You've completed the game, you know all the way. So, if not aiming for multiple achievements at once, this one takes about a hour or less.

Don't beat a single enemy.
Difficulty: Medium.

Beat the game without beating anyone. The most of enemies are easy to avoid, especially on Normal.

Also, it is allowed to stun them with the purple skill or dash with the blue ( !!! the exceptions are Gaz Beasts !!! don't use the purple + blue on them, because it often causes a never-fixed game crash bug).
Even the red hits and bombs are allowed as long as they are not last hits to beat an enemy. But I'd advise simply avoiding those.

If you have beaten some poor thing, exit to the menu and reload from a previous checkpoint.

This one is easy if done alone, much harder if you are aiming for multiple achievements at one go (yes, that was me).
Not combines with Happy Now?, KABOOM! and The Offering.

Beat the game with a purple Lumini.
Difficulty: Medium.

Purple Lumini spawn on the Checkpoint #1 only. So your mission is to save at least one of them until the end.

You automatically get this achievement when you go for Leave no Lumini behind.

Going for this one alone:

• You can always exit (to the menu or close the game) and reload a previous checkpoint in unlucky case if you've lost all of your purples.
  Just remember to have at least one of them before you hit the next checkpoint, since it automatically saves your progress with no revert.
  
  
• Get that lot of energy between the Checkpoints from #17 to #21 to grow your swarm big. After that, the last part, keep your purple in the middle of Lumini swarm - go with yellow. So even if something hits them, purples will be safe.
  
  
• If you feel like spawning more purples in the beginning - there are guides about that. However, I don't recommend having too many because the bigger your swarm the harder to save them. For the achievement, even one is enough. But it's up to you.

Don't lose a single Lumini.
Difficulty: Hard.

Do it on Normal, with less enemies.

Have all 4 Lumini colors by the end (see how to spawn different colors in Happy Now? part if you need).

Avoid taking extra energy, you want your swarm to be the smallest possible.
Collect just enough to activate every checkpoint - in my experience, this achievement doesn't work if you've passed some checkpoints with zero energy collected, also those don't save your progress ( I have no clue if this was the reason or the only reason, just my guess why it could be - but do you want to risk it? )

If you lost a Lumini, you can exit and reload from a previous checkpoint.

The hardest part is the last - after the Checkpoint #21. Be ready to retry this part multiple times, even with a little swarm it's hard to protect them all.
You can use the blue dash skill, hit with the red and purple - two times upgraded red is the best, but still it has a cooldown.

If your swarm is too long to protect with a skill radius, you can try to split it into two groups and move them very close to each other. Thus the swarm will be about twice shorter. All skills work for both groups at the same time. But since the key to move groups synchronously doesn't work, it requires to move them with two hands. Might work for you.

Be careful and good luck!

Alternatively, it is possible to use this bug to skip the rock fall activation trigger.

1.

(pics are clickable)
You can squeeze through the bugged floor textures, right before the blocking your way plants.

2.

Spend about 2 min to fly → ↑ ↗
through the mess of the backend side of the level. Thus avoiding the start of rockfall.

3.

Enter the level area later on and complete it like you would normally do, but without an avalanche killing your Lumini.

Not saying that this is the only way to get this achievement, but it exists.

The rest of the guide is all about collectibles, it covers the last 3 achievements.

The Walkthrough part about collectible locations combines them all together. If you're going for The Hoarder, you'll get the other two on the way.

And I would advise you to dedicate one playthrough only to collecting without being distracted by anything else. Because of The Hoarder. But of course it's up to you.

There are four types of objects to collect:

Fully evolve all Lumini types.
Difficulty: Easy.

Evolve blue, red and yellow Lumini 2 times each. In other words, you need to collect 6 out of 7 Upgrades in one playthrough.

Upgrades are at the Checkpoint #5, #7, #8, #9, #10, #13 and #14.
They are easy to find, as you can hear the sound-noise they make from afar. More info on their locations is in the Walkthrough section.

Any upgrade can be taken by blue, red or yellow at your choice.
Make sure you have at least one Lumini of the color you want to evolve when you approach another Upgrade.

Each Lumini color can evolve no more than 2 times.

Basic skills are blue - dash, red - hit, yellow - energy attracting.

Evolving any Lumini color, you see their new form and, more importantly, you upgrade their skill.

The basic function is always the same, but you get additional features like shields or a stunning dash trail. Play around to learn more.

Despite the fact that there is a purple upgrade crystal in the game, purple Lumini can't pick it up and don't have an upgraded form, so the seventh upgrade is useless. ¯\_(ツ)_/¯

Unlock all models and concept art in the Extras menu.
Difficulty: Easy.

Collect 20 crystals across all your playthroughs. They are easy to find by the sound they make and are just a little better hidden than Upgrades.

Note, you'll see a total of 24 on the counter displayed on your screen by collecting all 20 of them.
(4 of crystals give +2 to your counter? or 4 extras are just given to you on the way without collecting a crystal?... Anyway). ¯\_(ツ)_/¯

Crystals can be found:

• Checkpoint #1
  
• Checkpoint #2 - 2 crystals
  
• Checkpoint #3
  
• Checkpoint #4 - 2 crystals
  
• Checkpoint #5 - 2 crystals
  
• Checkpoint #6
  
• Checkpoint #7
  
• Checkpoint #9 - 3 crystals
  
• Checkpoint #10
  
• Checkpoint #11
  
• Checkpoint #12
  
• Checkpoint #13
  
• Checkpoint #14 - 2 crystals
  
• Checkpoint #15

More info on their locations is in the Walkthrough section.

Collect all energy.
Difficulty: Hard.

The biggest part of collecting energy is quite easy. Collect everything you can see: fly around all the area you are in, see all the sides including top, don't forget to fly around all interior smaller stones inside of big caves.

Do it without enemies around, so on Normal and when you're not going for The Pacifist.

Split the swarm of Lumini into two groups often and send them as far apart from each other as possible. Thus, you see a large area at once.

Your finding progress saved across all playthroughs. So you don't have to find it all at once, and you also don't have to recollect all the re-spawned energy within one playthrough.

Now about the hard part.
The hidden areas. With this guide you'll find them all.
But, unfortunately, there is no hint of how many Energy you still have to find, also no indication of what Energy has already been counted as collected earlier.
So you may still want to recollect everything in order from the beginning to the achievement.

Walkthrough section is dedicated to places you might miss.
Some of them are not hidden, but missable because either they are aside from your path or locked.
So if you have already carefully completed the "collect everything you can see" part (or doing it on the way by yourself), then you can only focus on the hidden areas - paths behind the stones and such. Those areas are marked with ⚠️ in the Walkthrough.

1. Get yellow Lumini. The first Yellow orbs can be found before the Checkpoint #4, so it should spawn your first yellow. And use the yellow skill.
   It helps you to attract Energy as a magnet, even if you don't touch it - it's very useful when the Energy flowers are hidden from your sight behind the stones, inside of hidden ways.
   
   Just be aware, it magnets Boom Bloom's thorns as well so clean out those first.
   
   
2. Keep them alive all the way - if you lose all yellows, you can't use the skill.
   
   
3. Evolve your yellow Lumini first of all.
   To do so, choose yellow Lumini before you fly to pick up an Upgrade, its crystal should be yellow dodecahedron then.
   The first Upgrade can be found at the Checkpoint #5.
   
   Yellow skill upgraded once will show you directions to the close Energy flowers outside of your screen - by a characteristic glow. It points you to all missable Energy in the area and to the most of hidden areas too.
   
   
4. The wider visible area - the longer pointing distance of the skill. Use two groups to see more.
   
   Upgraded twice yellow skill also gives a temporary shield to all Lumini every time Energy collected with it - not so critical for The Hoarder, so it's up to you what to upgrade next.
   

Collectibles in chronological order - all locked or hidden areas.
I skip clearly visible Energy on the way that is easy to get without any hints. Keep in mind, you still need to collect it all at least once.

This section divided to 7 parts - by area themes. Hopefully, it helps you navigate in the timeline.
As alternative, checkpoint numbering is there too, if you prefer counting.

I mark with ⚠️ - areas that you could miss, even if you normally flew around and collected everything visible. They are hidden behind the walls and such, the most missable let's say.

!!!

There are screenshots, a lot!!! of them down there ↓
(clickable if you want full versions)

• Checkpoint ##: 1 - 3
  
• Areas to check: 7
  (Hidden ⚠️ 3)
  
• Upgrades: 0
  
• Crystals: 4
  
• Yellow orbs: 3(x3)

By the end of this part you have:

• Lumini upgraded - none
  
• Extras counter - 5

• Checkpoint #4
  
• Areas to check: 5
  (Hidden ⚠️ 1)
  
• Upgrades: 0
  
• Crystals: 2
  
• Yellow orbs: 6(x3)

By the end of this part you have:

• Lumini upgraded - none
  
• Extras counter - 7

• Checkpoint ##: 5 - 6
  
• Areas to check: 9
  (Hidden ⚠️ 5)
  
• Upgrades: 1
  
• Crystals: 4
  
• Yellow orbs: 13(x3)

By the end of this part you have:

• Lumini upgraded - 1 time
  
• Extras counter - 11

• Checkpoint ##: 7 - 8
  
• Areas to check: 7
  (Hidden ⚠️ 2)
  
• Upgrades: 2
  
• Crystals: 1
  
• Yellow orbs: 12(x3)

By the end of this part you have:

• Lumini upgraded - 3 times
  
• Extras counter - 12

• Checkpoint ##: 9 - 12
  
• Areas to check: 23
  (Hidden ⚠️ 7)
  
• Upgrades: 2
  
• Crystals: 7
  
• Yellow orbs: 24(x3)

By the end of this part you have:

• Lumini upgraded - 5 times
  
• Extras counter - 19

• Checkpoint ##: 13 - 16
  
• Areas to check: 19
  (Hidden ⚠️ 4)
  
• Upgrades: 2
  
• Crystals: 5
  
• Yellow orbs: 11(x3)

By the end of this part you have:

• Lumini upgraded - all
  
• Extras counter - 24 (all)

Technically, it's mutation
The Art Collector

• Checkpoint ##: 17 - 21
  
• Areas to check: 2
  (Hidden ⚠️ 1)
  
• Upgrades: 0
  
• Crystals: 0
  
• Yellow orbs: 1(x3)

By the end of this part you have:

• all the energy

The Hoarder

Here is a conclusion of this guide.

If you're reading this, thanks! And maybe you really needed it if you got this far.

Any questions, comments, grammar police, pointers to confusing parts, or anything else you want to say about the guide - let me know in the comments. Everyone is welcome, everything is appreciated.

For whatever reason, I put quite a lot of work into this. I was able to finish this Stiltwalker in a month. So maybe it deserved an award?
A new cat sticker for me would be nice :3 Yes, I really doubt that anyone is reading this.

That's all.
Peace to you.