Full list of all 54 Enter the Gungeon achievements. It takes around 120-150 hours to unlock all of the achievements in the base game on Windows.
The base game contains 48 achievements, and there are 2 DLC packs containing 6 achievements.
Map the first Five Chambers for the lost adventurer
Populate the Breach
Clear the Third Chamber 30 Times
Kill a Jammed Boss
Clear the Fourth Chamber 20 Times
Ace Winchester's game 3 times
Die in the Past
Clear the Second Chamber 40 Times
Access the Temple
Unlock the Bullet
Complete the game with Beast Mode on
Clear the Fifth Chamber 10 Times
Open the shortcut to the Fifth Chamber
Clear the First Chamber 50 Times
Clear the Sixth Chamber
Complete all 4 main character Pasts
Kill the Pilot's Past
Kill the Marine's Past
Open the shortcut to the Fourth Chamber
Deliver the Golem's replacement arm
Kill the Convict's Past
Kill 100 enemies by knocking them into pits
Win a wager against the Gunsling King 5 times
Carry a large amount of money at once
Spend big at the Acquisitions Department
Kill the Hunter's Past
Construct the Bullet that can kill the Past
Fall at the last second
Unlock the Robot
Why
Collect five Master Rounds in one run
Open the shortcut to the Third Chamber
Defeat the Boss of the Fifth Chamber
You've met with a terrible fate, haven't you
Accessed the Hidden Market
Get the jump on a Mimic
Always be flipping. Guns are for flippers
Kill a boss after covering it with glitter
Open the shortcut to the Second Chamber
Kill a frozen enemy by rolling into it
Access the Oubliette
Avenge Manuel
Complete the Tutorial
Complete the game with the Sorceress's Enchanted Gun
Complete Frifle's Challenges
Steal 10 things
Kill 100 enemies by dropping chandeliers
Kill 100 enemies while riding in a mine cart
Complete Daisuke's trial
Kill the Robot's Past
Kill the Bullet's Past
Take Revenge
Defeat an Advanced Boss
Complete Tonic's Challenge