There were 36 achievements as of the making of this guide

• Almost all of the following achievements are to be done within one playthrough, which means you'll need at least 2 full playthroughs to complete all of the achievements
  
• I will try as much as possible not to spoil the story or the important/surprising stuff of the game, so anyone can go through the guide at any stage of progression through the game
  
• This guide isn't a walkthrough, the point is not to tell you how to pass a certain room, but to tell you about the achievements, and the achievements only

Easiest way to do it:

• You will probably have obtained this one without noticing it, but in case you didn't, all you have to do is jump on (not just step on, but break) 10 grey blocks

Easiest way to do it:

• You will encounter (and probably die to) monsters, lava and moving platforms through your playthrough. In order to get killed by shots and explosions, you need to respectively get hit directly by a laser beam (shot by the big monsters and Grigori), and get killed by the blast of the explosion of the laser beam (when it touches a surface)

Easiest way to do it:

• You need to telejump to the Treeman in The hermit and the Lake, on the upper left of the screen

Easiest way to do it:

• You can do this on almost any screen if you know how to jump just before being teleported
  
• This is like learning how to bike, you just need to find your way to learn it and it'll stay

Unlocking "This is Pointless" through this method will automatically unlock the other two

How to:

• In order for this to work, the game must be running, on the foreground of your tasks and not paused. It can be done both in windowed and fullscreen mode

Easiest way to do it:

• Go at the end of a room, near the exit (whether it is a door or just a screen change),
  open the steam overlay (Tab+Shift by default), make sure the game isn't paused, and do something from there for at least 8 minutes, or go grab a snack
  
• As soon as the "This is pointless" achievement pops up, go to the next room and "Never give up" should unlock

How to:

• Go through a screen to another a certain amount of times

Easiest way to do it:

• You can just walk back and forth at the end of the screen to the next one infinetly, until you get all of the above achievements

How to:

• Hit each one of the beams a hundred times with your teleport gun

Easiest way to do it:

• You will naturally encounter all three beams through a playthrough, just hit them as soon as you see them

How to:

• Make teleshots bounce back and forth between mirrors

Easiest way to do it:

• In the room with the treeman and the 2 mirrors facing each other, get up on the little area inbetween the two mirrors and mash the button to shoot teleshots really hard

Star Killer and Unstoppable are pretty simple and self-explanatory, but if you need help go check "A small step" and "And this is my weapon"

How to: Have a plan to kill everyone you meet

• The achievement description is misleading, you actually have to kill every single monster on every screen except for ghosts at least once, and not just kill one of each kind

Don't miss:

• All the monsters before the alien city. See "Daredevil"

How to:

• Reach the super secret place where the upgrade is located
  
• Obviously since this is an upgrade you need to already have obtained the gauss gun

Easiest way to do it:
In order to get it you'll have to get the gauss gun without touching any checkpoints and suicide once you have it in order to respawn at the start. See "Daredevil"

• From Block Chompers, find a way up to the secret cliff on the left side

• From the Secret cliff, telejump your way up to the upper right of the screen

This is actually surprisingly easy to do even though it might take a few retries. It is necessary to know how to do this in order to get "Swiftest Success"
It is not necessary to have the gauss gun upgrade to be doable

Easiest way to do it:

• When he gets low, hit him in quick succession
  
• Shoot 1 or 2 times then dodge his bullet, and shoot another 1 or 2 time to make him change side, repeat
  
• It takes 3 hits to make Grigori change side, and you need to make him change side 4 times before being able to kill it

How to:

• Jump up the little piece of terrain right before the entrance to the city

Easiest way to do it:

From the Junkyard Exit go up, and then jump as far as you can to the right

Once you land there jump to the far right once again and you should get the achievement

/!\ Doing "Daredevil" is mandatory to get the first core and the gauss gun upgrade, you just need to suicide once you obtained "Daredevil" and the gauss gun /!\

This achievement is actually not as hard as it looks !
It sure will take a lot of retries, but it took me only about 45 minutes by myself and without knowing of the "Shorcut", so you'll probably be able to do it much faster with a guide !

How to:

• Go from the start of the game to the inside of the Alien City without activating any checkpoints

Easiest way to do it:

• Turn "autosave" off in the options
  
• If you ever touch a checkpoint or die, quit without saving and restart from the start
  
• Make sure you know how to properly use the "Shortcut", because it's not only gonna save you time, but also a ♥♥♥♥♥♥♥♥ of dangerous place and checkpoints to not go through

Checkpoints to watchout for:

The next screen has a checkpoint on the first block, you need to jump from there in order to avoid it

There should be a gray block there, you need to jump over it from the right distance to be able to avoid the checkpoint near it

Unlocking "Swiftest Success" will automatically unlock the other two

How to:

• Go sanic fast

Easiest way to do it:

• See: "I always get nauseous after teleporting", "Daredevil" and "Shortcut"
  
• Stay calm, and restart from the last checkpoint using "exit without saving" if you fail
  
• There is no easiest way to do it, the best way to go is to know every single trick of the game

You have plenty enough time to do it !

I did way under 20 minutes, just by using what I learned from "I always get nauseous after teleporting" and "Shortcut", and focusing a little bit :D

First Health Capsule

Second Health Capsule

Third Health Capsule
It is visible from the secret cave where you get the gauss gun

How to:

• When on Leap of Faith, jump and immediatly strafe left
  
• If you fail, go forward without touching any checkpoints untli you can suicide and repeat

There actually are more than 11 cores throughout the game, but the 2 first ones (during the space-battle) are not accounted for by the steam achievement system

Core #0a and #0b (Don't count towards the achievement)

1st Core

• To the left of the spawn, past the 2 grey blocks
  
• Gauss Gun required to get there
  
• See "Daredevil"

2nd Core

3rd Core

4th Core

5th Core

6th Core

7th Core

8th Core

• Jump to the left of the alien house

9th Core

10th Core

11th Core

Easiest way to do it:

• Every single alien counts, including the Stone Alien, the Treemen and the Alien Guards. Grigori does not seem to count
  
• Some aliens are hidden or hard to reach, like the "..Thanks?" Treeman, the Stone Alien and some Aliens in the Alien City

Watch out not to miss those:

There are actually more than 87 accessible (I counted 88, but it might be more)
Unlocking 107% unlocks the other two

How to:

• You need to find every single screen in the game

Easiest way to do it:

• Go through all the cores, artifacts and secrets locations
  
• Telejump out of every single screen where it is possible
  
• Jump as far and wide as you can, and try to reach the extremities of every screen to see if there isn't an hidden screen beyond

Some examples of hidden screens: