This guide lists all the achievements that can be missed during your journey in Clair Obscur: Expedition 33. Only missable achievements will be discussed in detail, so read before playing to ensure you don't miss any. If you're looking for a complete guide covering all achievements, I recommend the guide by ErickaUnlimited (which I used for my own 100% completion)

Oficial description: Defeat the Mime in Lumière.
How to get:
While playing the Prologue, you can walk around Lumière before the event called Gommage. Find a strange mime on this map and defeat him, it's not hard but take your time and take the opportunity to learn more about combat, try to parry his attacks.

Why it's missable: After the event called Gommage, there will be several changes, and you can't fight him anymore.

Oficial description: With Gustave, use a fully charged Overcharge that Breaks an enemy.
How to get:
You probably won't lose that, but it's technically missable. Fighting with Gustave permits you to charge him using common attacks and skills, that charge can be seen on right side, you can spend this charge using you "Overcharge" skill. To get the achievement, wait until the marker reach 10/10 and use you Overcharge to break an enemy.

Why its missable: After the end of Act I, you will not be able to use Gustave, so you will never use his skills anymore.

Oficial description: Find all 33 music records.
How to get:
You can find a total of 33 music records during the game, to find them you will need to defeat specific enemies, find some of them at specific locations, or buy them from some specific merchants.

Why its missable: There's only two missable music record

• "Lumière": You get it from defeating the Mime found on the Prologue, mentioned before. So, if you defeat the Mime on Lumière and get the "A Peculiar Encounter" achievement, you have it. (Edit note: Even if this mime is a missable encounter, it's possible to obtain "Lumière" later, but as mime has his own achievement, fight him)
  
• "Lettre à Maelle": You get it from watching an optional cutscene during Act II, when you defeat the first Axon and return to the camp, select the option "Check up on others" in the fireplace menu and watch the cutscene. I recommend to "Check up on others" every time it's possible, so, if the game put you on the camp, check in the fireplace if it's possible.

Oficial description: Find all of the journals from prior expeditions.
How to get:
There's a total of 49 journal to find during the game, you can find them mostly by exploring the game. Some of them can be a little tricky to find, but with a guide it can be done easily.

Why its missable:
There's Only one missable journal, called "Survivor", in order to get it, you will need an item called "Old key", which can only be obtained on Prologue, after the Gommage but before the Expedition, after this you will never be able to get this key. After the Gommage, you will get a token to use on some merchants, find the lady in blue and use this token to get the old key, after that you won't miss any journal.

Oficial description: Reach relationship level 7 with Maelle.
How to get:
To increase relationship with characters, you will need to talk with them on the camp and spend some time with them (the game asks if you want to do that sometimes). Every time that you are on the camp, try to speak with everyone, spend time with a character increases the relationship level by 1.

Why its missable: To relationship level 7 with Maelle, there's one specific dialog that she asks you something really important and the game permits you to tell the truth or lie (what can be very reasonable in the context), but she knows the truth, so, DON'T LIE. If you lie you won't be able to increase the relationship level with her.

Oficial description: Finish all of the Nevron quests.
How to get:
During the game you will find white Nevrons, that aren't hostiles, talking with them they will ask your help, usually simple tasks. In order to get this achievement you will need to help all of them.

Why its missable: Every time you talk with those white Nevrons, the game permits you to fight them. But if you kill them before complete their quest, you won't be able to do all of the Nevrom quests. So, don't kill them, even if you finish the quest, keep them alive and at the end you can get a reward that will be way better than the rewards from the fights.

Edit note: There's a possibility to lose the achieviement if you came to see the last white nevrom too late (not sure about when exactly, but after act two), so I recommend to get this achievements before finish act two. Basically, try to explore the world and finish all nevrom quests before head to monolith.

I hope this guide could help someone to not miss any achievements of this game.