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