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