It has been confirmed that some achievements cannot be done unless you do a completely New Game at current version of the game (Ver. 494439) March 24, 2025
i am sure this will be patched out later, but be warned for these for now though:

• Standing Alone achievement >> once you called for help (white summon) that state is carried over to NG+, meaning you cannot get it unless you do a compete new run. Confirmed by @MrLuckless and @EL Truco
  
• The Challenger’s Journey / The Expert’s Journey >> in my testing i cannot change difficulty back to Normal from Easy, so it is very likely if you beat the game on Easy, you cannot go back to Normal in NG+ (not confirmed)

How long to get all achievement?
From articles it said 30-35hours to beat the game,
Took me around 50hours to complete it (with some help for the collectibles).
so i'd say 45-60 hours on average -- depending on each player skills, and if they get help with the collectibles or not.

Do you need to replay the game?
NO

Here's the most efficient way to beat the game and get 100%:

1. Beat the game without changing the difficulty to Easy (just play the game normally and do not opt to easy when the game prompts you when you are having a rough time) --You can change difficulty to easy from Normal, but not the other way around. Beating the game on Normal unlocks the achievement for easy.
   
2. DO NOT call help (the white summon) -- some people asking question about this. and to be fair, i cannot answer as i am not doing another full playthrough to test. sorry
   
3. When you get to the last stage (you will know it's the last one if you pay attention to the story).
   But if not, the last stage is Imperial Citadel, the last boss is called Ozma
   here's a screenshot of the last save location before the boss
   -- screenshot --
   
   Check your achievements list (from game, shift+tab) -- if you have 52/57, you should be good. (confirmed by @StatzKamikatze )
   
4. After you beat the game, you should have 55/57 achievement, just opt for a new game plus, and you will get another achievement, summon a helper and it's 57/57. Congratulations!

• if you still haven't use the AI summon helper (spirit of advocacy) then it's advisable to NOT use it until you beat the game in the first playthrough. Since there is an achievement related to that.
  
• easy leveling up (about first half of game can be brute forced by having your level much higher than the req level) to do so, you need loads of Lacrima, use the Bard set when exploring (you should get it for free after several missions). If you want to use it at higher level, here's how to get the blueprint
  https://www.youtube.com/watch?v=9hRurm3O9yI&ab_channel=Reflectiongames
  trading the excess item to Lacrima instead of dismantling would be recommended early on, AND if you have unlocked the bard merchant and have excess coin, you can buy some gear and trade to lacrima too.
  the Maximum level you can go right now is 200, you need 41,944,980 lacrimas for this.
  the Maximum stat is 99
  
• BE WARY of (possibly from demo) early on weapon or gear that has different modifier. ie. i have a blue lvl 22 weapon that has B+/B rating instead of the usual B/C rating.
  --screenshot--
  funny thing is i don't remember where i got it. and i have the great sword and spear one, but not the dual sword. did i dismantle or sold it by accident?
  since we can augment the weapons level, this make the weapon useful until end game if you invest on the right stat.
  --screenshot sample--
  
  i use this until endgame (the last 2 boss i swap to endgame spear gear set, since i can't get reliable stagger of them, i think this is skill/patience issue lol)
  
  
• Game works with random loot drop based of a loot table. so if you beat an enemy or open a chest, what you get is not guaranteed. You can farm a boss or savescum to get different loot (tho it won't matter much early on, since we can change the stat/level up the item) i had a chest give me unique ring, just to have the game crashed and upon reopening the same chest, i got rare ring instead lol.
  Some set needs specific item, usually these are boss drop, and usually the item is easy to identify, ie. hooves from centaur boss, witch tentacle from witch boss. etc.
  i don't have any issue with the set i want (using spear) but if you need help, here's a wiki to look for
  https://firstberserkerkhazan.wiki.fextralife.com/
  
• Continuing the note above, the jar guy sells boss drop for coins, about 3 of each. BUT these do not refresh!
  
• the game has 3 autosave backup, which you can recover save game if it's corrupted or mess up, you can acces this feature by going to load instead of continue and press the corresponding button to recover.
  
• BUT, if you want to play safe, here's the game save location
  %LOCALAPPDATA%\The First Berserker Khazan\Saved\SaveGames
  
• New game + retains all your progress (item, level, even crystal, jar, collections) and introduce harder enemy and legendary gears as normal drops.

Just play main mission and side mission and these should be automatically unlocked


Ruler of the Frozen Mountain
Defeated Yetuga


Combative Phantom of the Netherworld
Defeated Blade Phantom.


Berserk Dragon’s General
Defeated Viper. (got nerfed post launch)


Fallen Guardian
Defeated Volbaino.


Origin of the Mutation
Defeated Aratra.


Slayer in the Citadel
Defeated Rangkus.


Consumed by Chaos
Defeated Maluca.


No Turning Back…
Defeated Elamein.


Linon’s Beast
Defeated Shactuka.


Return of the Witch
Defeated Trokka.


Hermit
Defeated Bellerian.


Bloodthirsty Fiend
Defeated Skalpel.


Tragedy of Pell Los
Defeated Princess Ilyna.


Hismar the Berserk Dragon
Defeated Hismar.


Faded Memories
Defeated Reese.


Traces of Two
Learned of Andell and Seona’s story.


The Hidden Laboratory
Unveiled the mysteries of El Ravaca’s laboratory.


Desperate Escape
Rescued the wrongfully accused subordinates.


Reese’s Past
Discovered the secrets of Reese’s past.


Tool of the Netherworld
Obtained Charon’s Chains.


Trial of the Sword
Overcame the trial of the sword.


Human Xilence
Allowed Xilence to find peace at last.


Undying Loyalty
Learned of Yulian and Tristan’s story.


The Order
Discovered the fate of Princess Ilyna and Skalpel.


Pact
Formed a pact with Blade Phantom.


Traces of a Stranger
Discovered a collection of items.


Combat Essentials
Used a consumable for the first time.


Ambush
Defeated an enemy using a breakfall.


Finishing Strike
Used a brutal attack.


Piercing Power
Used a javelin.


Tears of the Soul
Retrieved Lacrima using the power of the ghost.


Upgrade Stats
Upgraded a stat.


Skilled Combatant
Learned a combat skill.


Soulstone Unleashed
Upgraded abilities by unleashing the power of Soulstones.


Skilled Blacksmith
Had Qazumaka craft gear.


Blacksmith’s Resolve
Had Qazumaka dismantle gear.


Special Scroll
Obtained a Special Scroll.


Artisanship
Crafted a special item.


Kaleido
Had Oksana alter attributes.


Remnants of the Soul
Had Oksana exact Lacrima.


Indomitable
Inherited gear stats.


Fully Equipped
Equipped a piece of gear on every slot.


Resourceful Merchant
Purchased an item from Duimuk.


Merchant’s Wisdom
Sold an item to Duimuk.


Awakened Ghost
Woke a dormant ghost from its slumber.


Danjin’s Jars
Found one of Danjin’s jarlings.


The Bard’s Lamentations
Learned about Digor’s hidden past.
sometimes you meet the merchant bard in mission maps, i think you need to meet him several times, i go tthis unlocked automatically at mission Fall of the Empire, when i entered the room he is in (i haven't even interact with him yet when this pops up)


A New Path
Embarked on the second journey.

Wandering Soul
Summoned the wandering Soul of the Rift.


Standing Alone
Reached the ending without summoning a Spirit of Advocacy.


Countless Tales
Completed all bonus missions.


The Challenger’s Journey
Reached the ending.
Beat the game on Easy


The Expert’s Journey
Reached the ending after a difficult journey.
Beat the game on Normal/Hard


Jar Enthusiast
Found all of Danjin’s jarlings.


Agent of the Netherworld
Destroyed every Soulstone.

Ozma’s Past
Discovered the truth about Ozma
collect all Collection Items. (To check which collectibles you’re missing, head to Khazan’s Memories from any Blade Nexus).


THE FIRST BERSERKER
Reached ‘THE FIRST BERSERKER’ ending.
Basically get all collectibles (items, jars, crystal) prior to fighting Ozma.
*spoiler* when you meet Daphrona and you can pick "I must know the truth" -- you're good to go. Safest way to get this is do all side missions and collect all collectibles,jars,crystals
thank you to @Khang Younot

here's an excerp from fextralife

• You'll need to collect all 15 Golden Collection Items. You will also unlock Ozma's Past Trophy once you have collected all of them. (To check which collectibles you’re missing, head to Khazan’s Memories from any Blade Nexus. There, you can review each mission and easily spot which ones still have uncollected items).
  
• You will also need to complete specific Bonus Missions. While the exact bonus missions needed are still unclear, it’s highly recommended to finish all Bonus Missions before tackling the final main mission, just to be safe. Completing every Bonus Mission will also reward you with the Countless Tales trophy.
  
• When you find Daphrona at Corrupted Lounge Blade Nexus, in Master of Chaos Mission, pick the "I must know the truth" line.
  
• Once you have done all the above, when you face Ozma, he will have 3 phases instead of 2. Defeat him to unlock this ending.

first of all, since this game does not have minimap, it's hard to guide without a video.
Luckily Fextralife has made a video guide for this

https://www.youtube.com/watch?v=6DIn7HN37Ds&ab_channel=Fextralife

since this is a steam guide, in theory you should be able to pull this guide in steam overlay (shift+tab) and watch his video.
i am not going to make a guide for this since some of it are hidden quite well, and screenshot or text will just confuse you. And that video guide is very well made, with timestamp already included.